#include <iostream>
#include <string>
#include <set>
using namespace std;
/*
S must be equal to either the first half or second half of the string.
*/
int main()
{
int N;
cin >> N;
string U;
cin >> U;
if(N % 2 == 0)
{
cout << "NOT POSSIBLE\n";
return 0;
}
int a = 0, b = N/2, c = N/2 + 1;
bool flag = 0;
//Check if the first floor(half) of the string is equal to S
int i = 0, j = b - 1;
int psbl1 = 1, psbl2 = 1;
int x = 0;
for(i = 0; i < N/2; i++)
{
j++;
if(U[i] == U[j])
{
x++;
continue;
}
if(flag)
{
psbl1 = 0;
break;
}
j++;
flag = 1;
if(U[j] == U[i])
{
x++;
}
}
if(x != N/2) psbl1 = 0;
//Check if the last floor(half) of the string is equal to S
flag = 0;
i = c;
j = -1;
x = 0;
for(i = c; i < N; i++)
{
j++;
if(U[i] == U[j])
{
//cout << i << ' ' << j << '\n';
x++;
continue;
}
if(flag)
{
psbl2 = 0;
break;
}
j++;
flag = 1;
if(U[j] == U[i])
{
x++;
}
}
//cout << x << '\n';
if(x != N/2) psbl2 = 0;
// cout << psbl1 << psbl2 << '\n';
if(!psbl1 && !psbl2)
{
cout << "NOT POSSIBLE\n";
}
else if(psbl1 && psbl2)
{
if(U.substr(0, N/2) == U.substr(c, N/2))
{
cout << U.substr(0, N/2) << '\n';
}
else cout << "NOT UNIQUE\n";
}
else if(psbl1)
{
cout << U.substr(a, N/2) << '\n';
}
else
{
cout << U.substr(c, N/2) << '\n';
}
}
