#include <bits/stdc++.h>
using namespace std;
string s;
const int mod = 100000000 + 7;
const int base = 131;
long long modpow(int b, int e) {
if(e == 0) return 1;
if(e & 1) return (modpow(b, e - 1) * b) % mod;
long long m = modpow(b, e >> 1);
return (m * m) % mod;
}
int main() {
int n;
cin >> n >> s;
n = s.size();
int sz = (n - 1) / 2;
auto func = [&] (int i, int j) {
long long pw = 1;
long long ans = 0;
for(int x = i; x <= j; x++) {
ans += pw * s[x];
pw = (pw * base) % mod;
ans %= mod;
}
return ans;
};
auto twice = [&] (int x) {
return (x * (modpow(base, sz) + 1)) % mod;
};
int p = func(0, sz - 1);
int q = func(sz + 1, n - 1);
p = twice(p); q = twice(q);
vector <long long> pre (n), suf (n);
long long var = 0;
for(int i = 0; i < n; i++) {
pre[i] = var;
var += modpow(base, i) * s[i];
var %= mod;
}
var = 0;
for(int i = n - 1; i >= 0; i--) {
suf[i] = var;
if(i == 0) break;
var += modpow(base, i - 1) * s[i];
var %= mod;
}
set <int> can;
int idx = -1;
for(int i = 0; i < n; i++) {
long long h = (pre[i] + suf[i]) % mod;
if(h == p || h == q) {
idx = i;
can.insert(h);
}
}
if(can.empty()) {
cout << "NOT POSSIBLE" << endl;
} else if (can.size() > 1) {
cout << "NOT UNIQUE" << endl;
} else {
string aux;
for(int i = 0; i < n; i++) {
if(idx != i) aux += s[i];
}
cout << aux.substr(0, sz) << endl;
}
return 0;
}
