#include <bits/stdc++.h>
using namespace std;
int n;
string s;
set<string> ans;
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
cin >> n >> s;
if(n%2==0){ cout << "NOT POSSIBLE"; return 0; }
if(s.substr(1,n/2)==s.substr(1+n/2,n/2)) ans.insert(s.substr(1,n/2));
bool sk = false,bm = true;
for(int i = 0,j = n/2;i < n/2 and j < n;)
{
if(s[i]!=s[j])
{
if(sk){ bm = false; break; }
else j++,sk = true;
}
else i++,j++;
}
if(bm) ans.insert(s.substr(0,n/2));
sk = false,bm = true;
for(int i = 0,j = n/2+1;i < n/2 and j < n;)
{
if(s[i]!=s[j])
{
if(sk){ bm = false; break; }
else i++,sk = true;
}
else i++,j++;
}
if(bm) ans.insert(s.substr(n/2+1,n/2));
if(ans.size()==0) cout << "NOT POSSIBLE";
else if(ans.size()>1) cout << "NOT UNIQUE";
else cout << *ans.begin();
}
