#include <bits/stdc++.h>
#define sz(x) (int)(x.size())
#define debug printf
#define lp(i,a,b) for(int i = a ; i < b; i++)
#define pb push_back
#define ff first
#define ss second
#define mk make_pair
#define pii pair<int,int>
#define ll long long
#define all(x) x.begin(),x.end()
using namespace std ;
int n ;
vector< string > possibleAnswers ;
void compare( vector<char> &part1, vector<char> &part2 )
{
int t = sz(part1) ;
if( sz(part2) != t+1 ) return ;
vector<bool> preffix(t+1,false) , suffix(t+1,false) ;
for(int i = 0 ; i < t ; i++ )
{
if( part1[i] != part2[i] ) break ;
preffix[i] = true ;
}
for(int i = t ; i > 0 ; i-- )
{
if(part1[i-1] != part2[i]) break ;
suffix[i] = true ;
}
for(int i = 0 ; i < t+1 ; i++ )
{
bool ok = true ;
if(i) ok &= preffix[i-1] ;
if(i < t) ok &= suffix[i+1] ;
if(!ok) continue ;
string str ;
for(auto e : part1 ) str.push_back(e) ;
possibleAnswers.pb(str) ;
return ;
}
}
void printAnswer(string str)
{
cout << str << endl ;
exit(0) ;
}
int main()
{
ios_base::sync_with_stdio(false) ;
cin.tie(NULL) ;
cin >> n ;
vector<char> str(n) ;
lp(i,0,n) cin >> str[i] ;
if(n%2 == 0) printAnswer("NOT POSSIBLE") ;
vector<char> part1, part2 ;
int t = (n-1)/2 ;
lp(i,0,t) part1.push_back(str[i]) ;
lp(i,t,n) part2.push_back(str[i]) ;
compare(part1, part2) ;
part1.clear() ;
part2.clear() ;
lp(i,0,t+1) part2.push_back(str[i]) ;
lp(i,t+1,n) part1.push_back(str[i]) ;
compare(part1, part2) ;
if(sz(possibleAnswers) == 0) printAnswer("NOT POSSIBLE") ;
if(sz(possibleAnswers) == 1) printAnswer(possibleAnswers[0]) ;
printAnswer("NOT UNIQUE") ;
}
