#include <bits/stdc++.h>
using namespace std;
const double PI = 3.14159265358979323846;
const long long inf = 2e9, MOD = 1e9 + 7, MAX = 1e4 + 1;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int n;
  string s;
  cin >> n >> s;
  for (int i = 0; i < n - 1; i++) {
    if (s[i] != s[i + 1])
      return cout << "YES"
                  << "\n"
                  << s[i] << s[i + 1],
             0;
  }
  cout << "NO";
}
