#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <math.h>
#include <string>
#include <algorithm>
#include <random>
#include <iomanip>
#include <utility>
#include <cstring>
//#include <bits/stdc++.h>
#define ll long long
#define s .size()
using namespace std;
const ll N=1e5+9,M=1e18+9,mod=1e9+7;
//cout<<fixed<<setprecision(6)<<
ll n,x,y;
vector<string>v;
string a,b,c,d,e,ans;
bool ok(){
y=d.size()/2,e="";
for(ll i=0;i<y;++i)
if(d[i]!=d[i+y])return 0;
else e+=d[i];
ans=e;
return 1;
}
int main()
{
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin>>n>>a;
if(!(n%2))goto end;
for(ll i=0;i<n;++i){
c="";
for(ll j=i+1;j<n;++j)c+=a[j];
d=b+c;
if(ok())++x;
if(x>1)break;
b+=a[i];
}
end:
if(!x)cout<<"NOT POSSIBLE";
else if(x>1)cout<<"NOT UNIQUE";
else cout<<ans;
}
