#include <bits/stdc++.h>
#define pb push_back
using namespace std;
int main()
{
int n, resp1 = 1, resp2 = 1;
string s;
cin >> n;
cin >> s;
if(n%2 == 0)
{
cout << "NOT POSSIBLE\n";
return 0;
}
int meio = n/2;
string a = "", b = "", s1 = "", s2 = "";
for(int i = 0 ; i < n ; i++)
{
if(i < meio) a.pb(s[i]);
else b.pb(s[i]);
}
int v = -1, j = 0;
for(int i = 0 ; i < meio ; i++)
{
if(a[i] != b[j])
{
if(v == -1 and a[i] == b[++j]) v = i;
else
{
resp1 = 0;
break;
}
}
s1.pb(a[i]);
j++;
}
v = -1, j = 0;
a = "", b = "";
for(int i = 0 ; i < n ; i++)
{
if(i <= meio) b.pb(s[i]);
else a.pb(s[i]);
}
for(int i = 0 ; i < meio ; i++)
{
if(a[i] != b[j])
{
if(v == -1 and a[i] == b[++j]) v = i;
else
{
resp2 = 0;
break;
}
}
s2.pb(a[i]);
j++;
}
if(!resp1 and !resp2) cout << "NOT POSSIBLE\n";
else if((resp1 and !resp2) or (resp1 and resp2 and s1 == s2)) cout << s1 << endl;
else if(resp2 and !resp1) cout << s2 << endl;
else cout << "NOT UNIQUE\n";
}
