// I stand with PALESTINE
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize ("unroll-loops")
//#pragma GCC target("avx,avx2")
#include <bits/stdc++.h>
//#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
//using namespace __gnu_pbds;
#define int long long
#define ff first
#define ss second
#define sz(a) (int) (a).size()
//template<class T, class C = null_type> using ordered_tree = tree<T, C, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
typedef long double ld;
namespace io {
template<typename F, typename S>
ostream &operator<<(ostream &os, const pair<F, S> &p) { return os << p.ff << " " << p.ss; }
template<typename F, typename S>
ostream &operator<<(ostream &os, const map<F, S> &mp) {
for (auto it: mp) { os << it << endl; }
return os;
}
template<typename F>
ostream &operator<<(ostream &os, const vector<F> &v) {
bool space = false;
for (F x: v) {
if (space) os << " ";
space = true;
os << x;
}
return os;
}
template<typename F>
ostream &operator<<(ostream &os, const deque<F> &d) {
bool space = false;
for (F x: d) {
if (space) os << " ";
space = true;
os << x;
}
return os;
}
template<typename F>
ostream &operator<<(ostream &os, const set<F> &st) {
bool space = false;
for (F x: st) {
if (space) os << " ";
space = true;
os << x;
}
return os;
}
template<typename F>
ostream &operator<<(ostream &os, const multiset<F> &st) {
bool space = false;
for (F x: st) {
if (space) os << " ";
space = true;
os << x << x;
}
return os;
}
template<typename F, typename S>
istream &operator>>(istream &is, pair<F, S> &p) { return is >> p.ff >> p.ss; }
template<typename F>
istream &operator>>(istream &is, vector<F> &v) {
for (F &x: v) { is >> x; }
return is;
}
long long fastread() {
char c;
long long d = 1, x = 0;
do c = getchar(); while (c == ' ' || c == '\n');
if (c == '-') c = getchar(), d = -1;
while (isdigit(c)) {
x = x * 10 + c - '0';
c = getchar();
}
return d * x;
}
static bool sep = false;
using std::to_string;
string to_string(bool x) {
return (x ? "true" : "false");
}
string to_string(const string &s) { return "\"" + s + "\""; }
string to_string(const char *s) { return "\"" + string(s) + "\""; }
string to_string(const char &c) {
string s;
s += c;
return "\'" + s + "\'";
}
template<typename Type>
string to_string(vector<Type>);
template<typename F, typename S>
string to_string(pair<F, S>);
template<typename Collection>
string to_string(Collection);
template<typename F, typename S>
string to_string(pair<F, S> p) { return "{" + to_string(p.ff) + ", " + to_string(p.ss) + "}"; }
template<typename Type>
string to_string(vector<Type> v) {
bool sep = false;
string s = "[";
for (Type x: v) {
if (sep) s += ", ";
sep = true;
s += to_string(x);
}
s += "]";
return s;
}
template<typename Collection>
string to_string(Collection collection) {
bool sep = false;
string s = "{";
for (auto x: collection) {
if (sep) s += ", ";
sep = true;
s += to_string(x);
}
s += "}";
return s;
}
void print() {
cout << endl;
sep = false;
}
template<typename F, typename... Other>
void print(F ff, Other... other) {
if (sep) cout << " | ";
sep = true;
cout << to_string(ff);
print(other...);
}
}
using namespace io;
namespace utils {
template<typename F, typename S>
inline void maxs(F &a, S b) { a = a > b ? a : b; }
template<typename F, typename S>
inline void mins(F &a, S b) { a = a < b ? a : b; }
template<typename F, typename S>
long long max(F a, S b) { return a > b ? a : b; }
template<typename F, typename S>
long long min(F a, S b) { return a < b ? a : b; }
constexpr long long operator "" _E(unsigned long long n) {
long long p = 1, a = 10;
for (int i = 0; i < n; i++) p *= a;
return p;
}
random_device rd;
mt19937 mt(rd());
template<typename T>
T rand(T l, T r) {
uniform_int_distribution<T> dist(l, r);
return dist(mt);
};
}
using namespace utils;
#ifdef sunnatov
#define print(...) cout << "[" << #__VA_ARGS__ << "]: "; io::print( __VA_ARGS__ );
#else
#define print( ... ) 42
#endif
const double EPS = 1e-7;
long long doxuya = 18_E + 10;
int INF = 9_E + 10;
const char nl = '\n';
int month[] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
/*
*/
vector<int> primes_list = {993146723,996752089,993912011,993276811,990439561,999134321,1000034863,990310561,1002852971,990902071,1002552547,1004791829,1006000901,1000242487,998810363,998510393,1005429353,1006591627,1007173513,1001089151,1007873257,1002742243,997222561,995034571,993148837,1007335169,1007688763,1000458911,995690771,1000963499,1004540941,996669341,1009791163,1005362731,990259279,1004949923,999257549,994083157,997000859,991488709,990129697,991686341,1006200047,995269721,998938153,1001274091,1008215713,1005259139,990913177,1000705361,992677943,1001761259,1008933127,991650763,1003413907,1001591489,999972499,1001013547,996769511,1001061553,998571107,1005498281,1004864467,1008537151,995490059,994271963,1000330937,1007601659,999935701,1007473871,1005960979,1009408901,1003306259,1001731277,993759673,1005202493,994289501,992293103,1007638627,991446553,991299677,1007753729,1003063247,1008357107,995452751,995169853,993065287,1009534639,991566083,995628827,992550289,993078701,1009667231,996338407,996761473,1008855797,1005879461,1000745609,997377253,1000759181};
int base, mod;
vector<int> modPow;
struct __hash {
vector<int> h;
inline void init(const vector<int> &a) {
if (base == 0) {
mt19937 rngHash(chrono::high_resolution_clock::now().time_since_epoch().count());
mod = primes_list[rngHash() % (int) primes_list.size()];
base = rngHash() % (mod - 2) + 2;
}
int n = (int) a.size();
h.resize(n);
int sz = (int) modPow.size();
if (sz <= n) {
modPow.resize(n + 1);
modPow[0] = 1;
for (int i = max(1, sz); i <= n; i++)
modPow[i] = 1ll * modPow[i - 1] * base % mod;
}
for (int i = 0; i < n; i++) {
h[i] = a[i] % mod;
if (i > 0) h[i] = (1ll * h[i - 1] * base + h[i]) % mod;
}
}
inline void init(const string &s) {
int n = (int) s.size();
vector<int> a(n);
for (int i = 0; i < n; i++) a[i] = (int) s[i];
init(a);
}
inline int get(int l, int r) {
assert(l <= r);
int res = h[r];
if (l > 0){
res -= (long long) modPow[r - l + 1] * h[l - 1] % mod;
if (res < 0) res += mod;
}
return res;
}
inline bool same(int l1, int r1, int l2, int r2) {
return get(l1, r1) == get(l2, r2);
}
};
void solution(istream &cin, ostream &cout, const int &test_case) {
int n;
string s;
cin >> n >> s;
if (n % 2 == 0) {
cout << "NOT POSSIBLE";
return;
}
__hash hsh; hsh.init(s);
auto [la, ra] = make_pair(-1, -1);
for (int i = 0; i < n; i++) {
vector<pair<int, int>> ranges;
if (i) ranges.emplace_back(0, i - 1);
if (i + 1 < n) ranges.emplace_back(i + 1, n - 1);
sort(ranges.begin(), ranges.end(), [&](pair<int, int> a, pair<int, int> b) {
return a.ss - a.ff > b.ss - b.ff;
});
if (sz(ranges) == 1) {
auto [l, r] = ranges[0];
if (hsh.get(l, (l + r) / 2) != hsh.get((l + r) / 2 + 1, r)) continue;
if (la == -1) la = l, ra = (l + r) / 2;
else if (hsh.get(la, ra) != hsh.get(l, (l + r) / 2)) {
cout << "NOT UNIQUE";
return;
}
continue;
}
auto [l, r] = ranges[0];
if (r == n - 1 and (r - l + 1) > n / 2) {
ranges.erase(ranges.begin());
ranges.emplace_back(r - n / 2 + 1, r);
ranges.emplace_back(l, r - n / 2);
} else if ((r - l + 1) > n / 2) {
ranges.erase(ranges.begin());
ranges.emplace_back(l, l + n / 2 - 1);
ranges.emplace_back(l + n / 2, r);
}
if (sz(ranges) == 2) {
auto [l, r] = ranges[0];
if (hsh.get(l, r) != hsh.get(ranges[1].ff, ranges[1].ss)) continue;
if (la == -1) tie(la, ra) = ranges[0];
else if (hsh.get(la, ra) != hsh.get(l, r)) {
cout << "NOT UNIQUE";
return;
}
continue;
}
sort(ranges.begin(), ranges.end());
print(i, ranges);
if (ranges[0].ss - ranges[0].ff > ranges[2].ss - ranges[2].ff) {
int ln = ranges[1].ss - ranges[1].ff + 1;
bool ok = hsh.get(0, ln - 1) == hsh.get(ranges[1].ff, ranges[1].ss);
ok &= hsh.get(ln, ranges[0].ss) == hsh.get(ranges[2].ff, ranges[2].ss);
if (!ok) continue;
if (la == -1) tie(la, ra) = ranges[0];
else if (hsh.get(la, ra) != hsh.get(ranges[0].ff, ranges[0].ss)) {
cout << "NOT UNIQUE";
return;
}
} else {
int ln = ranges[0].ss - ranges[0].ff + 1;
print(ranges[2].ff, ranges[2].ff + ln - 1);
bool ok = hsh.get(ranges[2].ff, ranges[2].ff + ln - 1) == hsh.get(ranges[0].ff, ranges[0].ss);
ok &= hsh.get(ranges[2].ff + ln, ranges[2].ss) == hsh.get(ranges[1].ff, ranges[1].ss);
if (!ok) continue;
if (la == -1) tie(la, ra) = ranges[2];
else if (hsh.get(la, ra) != hsh.get(ranges[2].ff, ranges[2].ss)) {
cout << "NOT UNIQUE";
return;
}
}
}
if (la == -1) {
cout << "NOT POSSIBLE";
return;
}
cout << s.substr(la, ra - la + 1);
}
int32_t main() {
clock_t startTime = clock();
cin.tie(0)->sync_with_stdio(false);
srand(time(0));
std::istream &in(std::cin);
std::ostream &out(std::cout);
int32_t queries = 1;
#ifdef test_cases
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
cin >> queries;
#else
// cin >> queries;
#endif
for (int32_t test_case = 1; test_case <= queries; test_case++) {
print(test_case);
solution(cin, cout, test_case);
cout << nl;
}
#ifdef sunnatov
cout << "Time: " << (int) ((double) (clock() - startTime) / CLOCKS_PER_SEC * 1000) << " ms" << endl;
#endif
return EXIT_SUCCESS;
}
