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