#include <bits/stdc++.h>
using namespace std;
#define int long long
#define all(A) A.begin(),A.end()
#define bp '\n'
#define vp cout<<'\n';
const int N=2e5+5;
int d[200];
signed main(){
cin.tie(nullptr)->sync_with_stdio(false);
string a,b,c;
int n=a.size(),cnt=0;
cin>>n;
cin>>a;
for(auto&e:a)c+=e;
string ans;
unordered_set<string>st;
for(int i=0;i<n;++i){
reverse(all(c));
c.pop_back();
reverse(all(c));
if(i>0)b+=a[i-1];
string h=b;
h+=c;
//cout<<b<<":"<<c<<' '<<h<<'\n';
int fl=1;
for(int i=0;i<(n-1)/2;++i){
// cout<<h[i]<<' '<<h[i+(n-1)/2]<<'\n';
if(h[i]!=h[i+(n-1)/2]){
fl=0;
break;
}
}
if(fl){
ans=h;
++cnt;
//st.emplace(h);
}
}
if(cnt==0 or n%2==0){
cout<<"NOT POSSIBLE";
}else if(cnt>1){
cout<<"NOT UNIQUE";
}else{
//cout<<ans;
for(int i=0;i<(n-1)/2;++i){
cout<<ans[i];
}
}
cout<<'\n';
}
/*
3
print
the
poem
*/
