#include <bits/stdc++.h>
using namespace std;
vector<size_t> prefixoprefixes(string S)
{
const size_t n = S.size();
vector<size_t> Z(n);
for(size_t i = 1, p = 0; i < n; i++)
{
if(i <= p + Z[p])
Z[i] = min(Z[i - p], p + Z[p] - i);
while(i + Z[i] < n and S[Z[i]] == S[i+Z[i]])
Z[i]++;
if(i + Z[i] > p + Z[p])
p = i;
}
return Z;
}
bool square_insert(string S)
{
auto P = prefixoprefixes(S);
auto Q = prefixoprefixes(string(S.rbegin(), S.rend()));
reverse(Q.begin(), Q.end());
const size_t n = S.size() / 2;
for(size_t i = 0; i <= n; i++)
if(P[n+1] >= i and Q[n] >= n - i)
return true;
return false;
}
int main()
{
ios::sync_with_stdio(false); cin.tie(nullptr);
size_t n;
cin >> n;
string S;
cin >> S;
if(n % 2 == 0)
{
cout << "NOT POSSIBLE" << endl;
return 0;
}
auto l = square_insert(S), r = square_insert(string(S.rbegin(), S.rend()));
auto L = S.substr(S.size()/2 + 1, S.size()/2), R = S.substr(0, S.size()/2);
if(l and r and L != R)
cout << "NOT UNIQUE" << endl;
else if(l)
cout << L << endl;
else if(r)
cout << R << endl;
else
cout << "NOT POSSIBLE" << endl;
}
