#include <bits/stdc++.h>
using namespace std;
int n;
string u;
map<char, int> ct;
int main()
{
cin >> n >> u;
if(n % 2 == 0)
{
cout << "NOT POSSIBLE\n";
return 0;
}
for(auto x: u) ct[x]++;
int I = 0;
char rep = '.';
for(auto x: ct)
if(x.second % 2)
I++, rep = x.first;
if(I > 1)
{
cout << "NOT POSSIBLE\n";
return 0;
}
int mid = (n-1)/2;
string s1, s2;
int ok = 2;
int id = -1;
for(int i = 0 ; i < mid ; ++i) s1.push_back(u[i]);
for(int i = mid ; i < n ; ++i) s2.push_back(u[i]);
for(int i = 0, j = 0, k = 0; i < mid ; ++j, ++i)
{
if(s1[i] != s2[j])
{ if(k == 0 && s1[i] == s2[j+1]) id = j, j++, k = 1;
else
{
ok--;
id = -1;
break;
}
}
}
s1.clear(); s2.clear();
for(int i = 0 ; i <= mid ; ++i) s2.push_back(u[i]);
for(int i = mid + 1 ; i < n ; ++i) s1.push_back(u[i]);
/* for(int i = 0 ; i < s2.size() ; ++i) cout << s2[i];
cout << "\n";
for(int i = 0 ; i < s1.size() ; ++i) cout << s1[i];
*/
for(int i = 0, j = 0, k = 0; i < mid ; ++j, ++i)
{
if(s1[i] != s2[j])
{ if(k == 0 && s1[i] == s2[j+1]) id = j, j++, k = 1;
else
{
ok--;
id = -1;
break;
}
}
}
if(ok == 0) cout << "NOT POSSIBLE\n";
else if(ok == 1 || ct[rep] == n)
{
if(id <= mid && id != -1) mid++;
for(int i = 0 ; i < mid ; ++i) if(i != id) cout << u[i];
cout << "\n";
}else cout << "NOT UNIQUE\n";
return 0;
}
