#include <bits/stdc++.h>
using namespace std;
char s[1005] = {0};
int k[26] = {0};
int main() {
  int n, p = 0;
  cin >> n;
  cin >> s;
  for (int i = 0; i < n - 1; i++) {
    if (s[i] != s[i + 1]) {
      p = 1;
      cout << "YES" << endl;
      cout << s[i] << s[i + 1] << endl;
      break;
    }
  }
  if (p == 0) cout << "NO" << endl;
  return 0;
}
