#include <bits/stdc++.h>
#define N 2050
using namespace std;
int n;
set<string> qtd;
string s, best;
string get(int ini, int fim, int block)
{
string ss = "";
for(int i = ini; i <= fim; i++)
if(i != block) ss.push_back(s[i]);
return ss;
}
int main()
{
ios::sync_with_stdio(false); cin.tie(0);
cin>>n;
cin>>s;
if(s.size() % 2 == 0)
{
cout<<"NOT POSSIBLE\n";
return 0;
}
for(int p = 0; p < n; p++)
{
int l1 = 0, r1 = (n/2) - 1;
int l2 = (n/2) + 1, r2 = n - 1;
if(p <= r1) r1 ++;
else l2 --;
string esq = get(l1, r1, p), dir = get(l2, r2, p);
if(esq == dir) best = esq, qtd.insert(esq);
}
if(qtd.size() == 0) cout<<"NOT POSSIBLE\n";
else if(qtd.size() == 1) cout<<best<<'\n';
else cout<<"NOT UNIQUE\n";
}
