#include <bits/stdc++.h>
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define FORd(i, a, b) for (int i = (a); i >= (b); i--)
#define REP(i, n) FOR(i, 0, n)
#define ll long long
using namespace std;
const int MAXN = 2001000;
ll ha[MAXN], pot[MAXN];
const int BASE = 35537;
// const int MOD = 1e9 + 7;
ll val(int x, int y) {
if (y < x) return 0;
ll out = ha[y];
if (x) out = out - ha[x - 1] * pot[y - x + 1];
return out;
}
int main() {
ios_base::sync_with_stdio(false);
pot[0] = 1;
FOR(i, 1, MAXN) pot[i] = pot[i - 1] * BASE;
int n;
cin >> n;
string s;
cin >> s;
ll curr = 0;
REP(i, n) {
curr = curr * BASE + s[i];
ha[i] = curr;
}
string out = "";
ll V = -1;
REP(i, n) {
if (i <= n / 2) {
ll a = val(0, i - 1) * pot[n / 2 - i] + val(i + 1, n / 2);
ll b = val(n / 2 + 1, n - 1);
if (a != b) continue;
string curr = s.substr(n / 2 + 1, n / 2);
if (out != "" && V != a) {
cout << "NOT UNIQUE\n";
return 0;
} else {
V = a;
out = curr;
}
} else {
ll a = val(0, n / 2 - 1);
ll b = val(n / 2, i - 1) * pot[n - i - 1] + val(i + 1, n - 1);
if (a != b) continue;
string curr = s.substr(0, n / 2 - 1);
if (out != "" && V != a) {
cout << "NOT UNIQUE\n";
return 0;
} else {
V = a;
out = curr;
}
}
}
if (out == "") cout << "NOT POSSIBLE\n";
else cout << out << "\n";
return 0;
}
