#include <bits/stdc++.h>
#define N 2000050
using namespace std;
int n;
string s, A, B, C, D;
set< pair<string, char> > resp;
int main()
{
ios::sync_with_stdio(false); cin.tie(0);
cin>>n;
cin>>s;
if(n%2 == 0)
{
cout<<"NOT POSSIBLE\n";
return 0;
}
for(int i = 0; i < n/2; i++) A.push_back(s[i]);
for(int i = n/2; i < n; i++) B.push_back(s[i]);
for(int i = n/2 + 1; i < n; i++) C.push_back(s[i]);
for(int i = 0; i < n/2 + 1; i++) D.push_back(s[i]);
// Checar se a resposta é "A"
int a = (int)A.size() - 1, b = (int)B.size() - 1;
//cout<<A<<" "<<B<<"\n";
int dif = 0, t = 0;
while(a >= 0 and b >= 0)
{
if(A[a] != B[b])
{
t = b;
dif ++;
b --;
}
else a--, b--;
}
if(!dif) dif = 1;
if(dif == 1) resp.insert({A, B[t]});
//cout<<a<<" "<<b<<" "<<B[t]<<" "<<dif<<"\n";
// CHECAR SE "C" É RESPOSTA
int c = (int)C.size() - 1, d = (int)D.size() - 1;
//cout<<C<<" "<<D<<"\n";
int dif2 = 0, t2 = 0;
while(c >= 0 and d >= 0)
{
if(C[c] != D[d])
{
t2 = d;
dif2 ++;
d --;
}
else c--, d--;
}
if(!dif2) dif2 = 1;
if(dif2 == 1) resp.insert({C, D[t2]});
if(resp.size() == 0) cout<<"NOT POSSIBLE\n";
else if(resp.size() == 1) cout<<(*resp.begin()).first<<"\n";
else cout<<"NOT UNIQUE\n";
//cout<<c<<" "<<d<<" "<<D[t2]<<" "<<dif2<<"\n";
}
