#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  int n, pos;
  string s;
  cin >> n >> s;
  bool flag = false;
  for (int i = 1; i < n; i++) {
    if (s[i] == s[i - 1])
      continue;
    else {
      flag = true;
      pos = i;
      break;
    }
  }
  if (flag)
    cout << "YES" << endl, cout << s[pos - 1] << s[pos] << endl;
  else
    cout << "NO" << endl;
  return 0;
}
