#include <bits/stdc++.h>
#define ll long long
using namespace std;
ll dem=0;
void solve(string sub, string s){
sub=sub+sub;
ll i=0, j=0, d=0;
char e;
while(i<sub.size()&&j<s.size()){
if(sub[i]!=s[j]){
d++;
e=s[j];
i--;
}
i++;
j++;
if(d>=2){
return ;
}
}
//cout<<e<<' '<<sub<<endl;
dem++;
ll pos=sub.find(e);
if(pos<sub.size()) dem++;
}
ll i, j, n, dem2;
string s, s1, s2, sub;
int main(){
cin>>n;
cin.ignore();
getline(cin, s);
if(s.size()%2==0){
cout<<"NOT POSSIBLE";
return 0;
}
for(i=0; i<s.size()/2; i++){
s1+=s[i];
}
for(i=s.size()/2+1; i<s.size(); i++){
s2+=s[i];
}
//cout<<s1<<' '<<s2;
solve(s1, s);
//cout<<dem<<endl;
sub=s1;
if(dem>=2){
cout<<"NOT UNIQUE";
return 0;
}
ll dem2=dem;
solve(s2, s);
if(dem>=2) cout<<"NOT UNIQUE";
else if(dem==1&&dem2==1) cout<<sub;
else if(dem==1&&dem2==0) cout<<s2;
else cout<<"NOT POSSIBLE";
}
//abcdefghg
