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