#include <bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
string s, a = "", b = "", answer1 = "", answer2 = "";
int c = 0;
cin >> s;
if (n % 2 == 0){cout << "Not Possible"; return 0;}
for (int i = 0; i < n/2; i++){
if (s[i] == s[n/2 + i + c]) {
a += s[i], b += s[n/2 + i + c];
}
else{
if (s[n/2 + i + 1] != s[i]){a = 'z', b = 'x'; break;}
else a += s[i], b += s[n/2 + i + 1], c = 1;
}
//cout << "a: " << a << " b: " << b << "\n";
}
if (a == b) answer1 = a;
//cout << "answer1: " << answer1 << "\n";
a = "", b = "", c = 0;
for (int i = 0; i < n/2; i++){
if (s[i + c] == s[n/2 + i + 1]){
a += s[i + c], b += s[n/2 + i + 1];
}
else{
if (s[i + 1] != s[n/2 + i + 1]){a = 'z', b = 'x'; break;}
else a += s[i + 1], b += s[n/2 + i + 1], c = 1;
}
//cout << "a: " << a << " b: " << b << "\n";
}
if (a == b) answer2 = b;
//cout << "answer2: " << answer2 << "\n";
if (answer1.size() && answer2.size() && answer1 != answer2) cout << "Not Unique";
else if (!answer1.size() && !answer2.size()) cout << "Not Possible";
else if (answer1.size()) cout << answer1;
else cout << answer2;
return 0;
}
