//thatsramen
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define eb emplace_back
#define pb push_back
#define ft first
#define sd second
#define pi pair<int, int>
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define dbg(...) dbg_out(__VA_ARGS__)
using ll = long long;
using ld = long double;
using namespace std;
using namespace __gnu_pbds;
//Constants
const ll INF = 5 * 1e18;
const int IINF = 2 * 1e9;
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
const ll dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
const ld PI = 3.14159265359;
//Templates
template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) {return os << '(' << p.first << ", " << p.second << ')';}
template<typename T_container, typename T = typename enable_if<!is_same<T_container, string>::value, typename T_container::value_type>::type> ostream& operator<<(ostream &os, const T_container &v) {os << '['; string sep; for (const T &x : v) os << sep << x, sep = ", "; return os << ']';}
void dbg_out() {cerr << endl;}
template<typename Head, typename... Tail> void dbg_out(Head H, Tail... T) { cerr << H << ' '; dbg_out(T...); }
template<typename T> void mins(T& x, T y) {x = min(x, y);}
template<typename T> void maxs(T& x, T y) {x = max(x, y);}
template<typename T> using oset = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T> using omset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;
//order_of_key(k): number of elements strictly less than k
//find_by_order(k): k-th element in the set
void setPrec() {cout << fixed << setprecision(15);}
void unsyncIO() {cin.tie(0)->sync_with_stdio(0);}
void setIn(string s) {freopen(s.c_str(), "r", stdin);}
void setOut(string s) {freopen(s.c_str(), "w", stdout);}
void setIO(string s = "") {
unsyncIO(); setPrec();
if(s.size()) setIn(s + ".in"), setOut(s + ".out");
}
//#define TEST_CASES
void solve() {
int n;
cin >> n;
string init;
cin >> init;
if (n % 2 == 0) {
cout << "NOT POSSIBLE";
return;
}
int len = n / 2;
auto check = [&](string s, string t) -> bool {
vector<int> left(n + 10, 0), right(n + 10, 0);
vector<int> pre(n + 10, 0), suf(n + 10, 0);
int s_size = s.size(), t_size = t.size();
{
string nw = s; nw += '#'; nw += t;
int m = nw.size();
for (int i = 1; i < m; i++) {
int j = pre[i - 1];
while (j > 0 && nw[j] != nw[i]) {
j = pre[j - 1];
}
if (nw[j] == nw[i]) {
pre[i] = j + 1;
} else {
pre[i] = 0;
}
if (i > s_size) {
int id = i - s_size - 1;
left[id] = pre[i];
}
}
}
{
reverse(all(s)); reverse(all(t));
string nw = s; nw += '#'; nw += t;
int m = nw.size();
for (int i = 1; i < m; i++) {
int j = suf[i - 1];
while (j > 0 && nw[j] != nw[i]) {
j = suf[j - 1];
}
if (nw[j] == nw[i]) {
suf[i] = j + 1;
} else {
suf[i] = 0;
}
if (i > s_size) {
int id = t_size - i + s_size;
right[id] = suf[i];
}
}
reverse(all(s)); reverse(all(t));
}
if (left[s_size - 1] == s_size || right[1] == s_size) {
return true;
}
for (int i = 1; i < t_size - 1; i++) {
if (left[i - 1] == i && right[i + 1] == t_size - (i + 1)) {
return true;
}
}
return false;
};
set<string> ans;
{
string s = init.substr(0, len);
string t = init.substr(len);
if (check(s, t)) ans.insert(s);
}
{
string s = init.substr(n - len, len);
string t = init.substr(0, n - len);
if (check(s, t)) ans.insert(s);
}
if ((int) ans.size() == 0) {
cout << "NOT POSSIBLE";
} else if ((int) ans.size() == 1) {
cout << (*ans.begin());
} else {
cout << "NOT UNIQUE";
}
}
int main() {
setIO();
int tt = 1;
#ifdef TEST_CASES
cin >> tt;
#endif
while (tt--)
solve();
return 0;
}
