#include "bits/stdc++.h"
using namespace std;
#ifdef duc_debug
#include "bits/debug.h"
#else
#define debug(...)
#endif
const int maxn = 2e5 + 5;
const int mod = 1e9 + 9;
const int base = 31;
int n;
string s;
int h[maxn], p[maxn];
void solve() {
cin >> n >> s;
if (n % 2 == 0) {
cout << "NOT POSSIBLE\n";
return;
}
s = ' ' + s;
for (int i = 1; i <= n; ++i) {
h[i] = ((1ll * h[i - 1] * base % mod) + (s[i] - 'A' + 1)) % mod;
}
auto get_hash = [&](int l, int r) -> int {
if (l > r) return 0;
return (h[r] - (1ll * h[l - 1] * p[r - l + 1] % mod) + mod) % mod;
};
vector<int> pos;
for (int i = 1; i <= n; ++i) {
int hh = 0;
if (i <= n / 2) {
hh = get_hash(n / 2 + 2, n);
} else {
hh = h[n / 2];
}
int rm = (1ll * get_hash(1, i - 1) * p[n - i] % mod + get_hash(i + 1, n)) % mod;
hh = (1ll * hh * p[n / 2] % mod + hh) % mod;
if (rm == hh) {
pos.push_back(i);
}
}
if (pos.empty()) {
cout << "NOT POSSIBLE\n";
} else if ((int)pos.size() > 1) {
cout << "NOT UNIQUE\n";
} else {
s.erase(s.begin() + pos[0]);
for (int i = 1; i <= n / 2; ++i) {
cout << s[i];
}
}
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
p[0] = 1;
for (int i = 1; i < maxn; ++i) {
p[i] = 1ll * p[i - 1] * base % mod;
}
solve();
return 0;
}
