#include <bits/stdc++.h>
using namespace std;
long long int gcd(int a, int b) { return (b == 0 ? a : gcd(b, a % b)); }
int main() {
  int T = 1;
  while (T--) {
    long long int N, M, i, j = 1, k = 0, x, y, z, inp, flag = 1, sum = 0,
                           count = 0, max = 0, r = 0, a, b, c;
    cin >> N;
    string str;
    cin >> str;
    for (i = 0; i < N - 1; i++) {
      if (str[i] != str[i + 1]) {
        cout << "YES" << endl;
        cout << str[i];
        cout << str[i + 1];
        return 0;
      }
    }
    cout << "NO" << endl;
  }
  return 0;
}
