#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define int long long
const int Mod = 998244353, P = 43;
signed main(){
int n; cin >> n;
string s; cin >> s;
if ( n % 2 == 0 ){
cout << "NOT POSSIBLE\n";
return 0;
}
vector <int> pw(1, 1);
while ( (int)pw.size() <= n ){
pw.pb(pw.back() * 1LL * P % Mod);
}
vector <int> p(n + 1);
for ( int i = 0; i < n; i++ ){
p[i + 1] = (p[i] * 1LL * P + s[i] - 'A' + 1) % Mod;
}
auto get = [&](int l, int r){
return ((p[r + 1] - p[l] * 1LL * pw[r - l + 1] % Mod) + Mod) % Mod;
};
int j = -1, k = n / 2, hash = 0;
for ( int i = 0; i < n; i++ ){
int x = 0, y = 0;
if ( i >= k ){
x = get(0, k - 1);
y = (get(n - k - 1, i - 1) * 1LL * pw[n - i - 1] + get(i + 1, n - 1)) % Mod;
} else{
y = get(n - k, n - 1);
x = (get(0, i - 1) * 1LL * pw[k - i] + get(i + 1, k)) % Mod;
}
if ( x == y ){
if ( j != -1 && hash != x ){
cout << "NOT UNIQUE\n";
return 0;
}
j = i, hash = x;
}
}
if ( j == -1 ){
cout << "NOT POSSIBLE\n";
return 0;
}
string ans;
for ( int i = 0; i < n; i++ ){
if ( i != j ) ans += s[i];
}
for ( int i = 0; i < k; i++ ){
cout << ans[i];
}
cout << '\n';
}
