#include <bits/stdc++.h>
using namespace std ;
#define int long long
int32_t main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n ; cin >> n ;
string s ; cin >> s ;
int counter = 0 ;
string ans ;
for ( int i = 0 ; i < n ; i++ )
{
string g = s ;
g.erase(g.begin()+i) ;
int x = (n-1)/2 ;
string a = g.substr(0,x) ;
string b = g.substr(x) ;
//cout << g << " " << a << " " << b << endl ;
if ( a == b )
{
counter++ ;
ans = a ;
}
}
if ( counter == 0 ) cout << "NOT POSSIBLE" << endl ;
else if ( counter > 1 ) cout << "NOT UNIQUE" << endl ;
else cout << ans << endl ;
}
