#include <bits/stdc++.h>
using namespace std;
string t;
int n;
int process( int k ) {
string l, r;
for( int i = 0 ; i < k ; i++ ) l += t[i];
for( int i = k ; i < n ; i++ ) r += t[i];
int p1 = 0, p2 = 0, len1 = l.length(), len2 = r.length();
//cout << l << " " << r << endl;
while( p1 < len1 && p2 < len2 && l[p1] == r[p2] ) {
p1++, p2++;
}
if( p1 == len1 ) return len1 + len2;
else if( p2 == len2 ) return len1;
int now;
if( k == n/2 ) {
now = len1 + p2;
p2++;
}
else {
now = p1;
p1++;
}
//printf("%d %d\n",p1,p2);
for( ; p1 < len1 && p2 < len2 ; p1++, p2++ ) {
//cout << l[p1] << " " << r[p2] << endl;
if( l[p1] != r[p2] ) return 0;
}
return now;
}
int main()
{
ios_base::sync_with_stdio(false);cin.tie(0);cerr.tie(0);cout.tie(0);
cin >> n >> t;
if( n % 2 == 0 ) return !printf("NOT POSSIBLE");
char a = t[0], b = t[1];
int chk = 1;
for( int i = 2 ; i < n ; i++ ) {
if( i % 2 == 0 && t[i] != a ) chk = 0;
else if( i % 2 && t[i] != b ) chk = 0;
}
if( chk ) return !printf("NOT UNIQUE");
int x = process( n/2 ), y = process( n/2 + 1 );
if( x ) {
//printf("%d\n",x);
int cnt = 0;
for( int i = 0 ; cnt < n/2 ; i++ ) {
if( i == x ) continue ;
cout << t[i];
cnt++;
}
return 0;
}
else if( y ) {
//printf("%d\n",y);
int cnt = 0;
for( int i = 0 ; cnt < n/2 ; i++ ) {
if( i == y ) continue ;
cout << t[i];
cnt++;
}
return 0;
}
printf("NOT POSSIBLE");
return 0;
}
