#include <bits/stdc++.h>
#define int long long
using namespace std;
namespace std {
template <typename T> ostream &operator<<(ostream &out, const vector<T> &vec) {
out << "[";
for (int i = 0; i < (int)vec.size(); ++i) {
out << vec[i];
if (i + 1 < (int)vec.size())
out << ", ";
}
return out << "]";
}
} // namespace std
void dbg_out() { cout << endl; }
template <typename Head, typename... Tail> void dbg_out(Head H, Tail... T) {
cout << ' ' << H;
dbg_out(T...);
}
#ifdef DEBUG
#define dbg(...) cout << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
#else
#define dbg(...)
#endif
/*
* sa[i] = starting index of i-th smallest suffix
* sa.len() = n+ 1, sa[0] = n
* lcp[i] = lcp(sa[i], sa[i-1]), lcp[0] = 0, len(sa) = n+1
*/
template <class T> struct RMQ {
vector<vector<T>> jmp;
RMQ(const vector<T> &V) : jmp(1, V) {
for (int pw = 1, k = 1; pw * 2 <= (int)V.size(); pw *= 2, ++k) {
jmp.emplace_back((int)V.size() - pw * 2 + 1);
for (int j = 0; j < (int)jmp[k].size(); ++j)
jmp[k][j] = min(jmp[k - 1][j], jmp[k - 1][j + pw]);
}
}
T query(int a, int b) { // [a, b)
assert(a < b);
int dep = 31 - __builtin_clz(b - a);
return min(jmp[dep][a], jmp[dep][b - (1 << dep)]);
}
};
struct SuffixArray {
vector<int> sa, lcp, rank;
string s;
RMQ<int> rmq;
SuffixArray(string &_s, int lim = 256) : rmq({}) {
s = _s;
int n = s.size() + 1, k = 0, a, b;
vector<int> x(s.begin(), s.end() + 1), y(n), ws(max(n, lim));
sa = lcp = rank = y, iota(sa.begin(), sa.end(), 0);
for (int j = 0, p = 0; p < n; j = max(1LL, 2 * j), lim = p) {
p = j, iota(y.begin(), y.end(), n - j);
for (int i = 0; i < n; ++i)
if (sa[i] >= j)
y[p++] = sa[i] - j;
fill(ws.begin(), ws.end(), 0);
for (int i = 0; i < n; ++i)
ws[x[i]]++;
for (int i = 1; i < lim; ++i)
ws[i] += ws[i - 1];
for (int i = n; i--;)
sa[--ws[x[y[i]]]] = y[i];
swap(x, y);
p = 1, x[sa[0]] = 0;
for (int i = 1; i < n; ++i)
a = sa[i - 1], b = sa[i],
x[b] = (y[a] == y[b] and y[a + j] == y[b + j]) ? p - 1 : p++;
}
for (int i = 1; i < n; ++i)
rank[sa[i]] = i;
for (int i = 0, j; i < n - 1; lcp[rank[i++]] = k)
for (k &&k--, j = sa[rank[i] - 1]; s[i + k] == s[j + k]; k++)
;
rmq = RMQ<int>(lcp);
}
int getLCP(int i, int j) {
if (i == j)
return s.size() - i;
i = rank[i];
j = rank[j];
if (i > j)
swap(i, j);
return rmq.query(i + 1, j + 1);
}
};
bool get(string U) {
int N = U.size() / 2;
SuffixArray suffix(U);
for (int skip = N; skip < 2 * N + 1; ++skip) {
if (suffix.getLCP(0, N) < skip - N)
continue;
if (skip < 2 * N and suffix.getLCP(skip - N, skip + 1) < N - (skip - N))
continue;
return true;
}
return false;
}
signed main(void) {
ios_base::sync_with_stdio(false);
cin.tie(0);
int N;
string U;
cin >> N >> U;
if (N % 2 == 0) {
cout << "NOT POSSIBLE\n";
return 0;
}
string T = U;
reverse(T.begin(), T.end());
bool okL = get(U);
bool okR = get(T);
N /= 2;
string L = U.substr(0, N);
string R = U.substr(N + 1, N);
dbg(L, R);
dbg(okL, okR);
if (!okL and !okR) {
cout << "NOT POSSIBLE\n";
return 0;
}
if (okL and okR and L != R) {
cout << "NOT UNIQUE\n";
return 0;
}
cout << (okL ? L : R) << endl;
}
