#include <bits/stdc++.h>
using namespace std;
int main() {
  long long k, flag = 0, i;
  char a[2000], k1, k2;
  cin >> k;
  cin >> a;
  if ((k == 1))
    cout << "NO\n";
  else {
    for (i = 0; i < k - 1; i++) {
      if ((a[i] != a[i + 1])) {
        flag = 1;
        k1 = a[i];
        k2 = a[i + 1];
        break;
      }
    }
    if (flag == 1) {
      cout << "YES\n";
      cout << k1 << k2 << endl;
    } else
      cout << "NO\n";
  }
  return 0;
}
