#include <bits/stdc++.h>
using namespace std;
template<typename T>
void out(T x) { cout << x << endl; exit(0); }
#define watch(x) cout << (#x) << " is " << (x) << endl
using ll = long long;
const int maxn = 1e6 + 5;
void notp() {
out("NOT POSSIBLE");
}
void notu() {
out("NOT UNIQUE");
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int n;
string u;
cin>>n>>u;
if (n%2==0) notp();
string ans;
for (int i=0; i<n; i++) {
string tmp = u.substr(0,i) + u.substr(i+1);
string s = tmp.substr(0,(n-1)/2);
string t = tmp.substr((n-1)/2);
if (s==t) {
if (ans!="") notu();
ans = s;
}
}
cout<<ans<<endl;
return 0;
}
