#include <bits/stdc++.h>
using namespace std;
char str[1009];
int main() {
  int n;
  cin >> n;
  cin >> str;
  int i, j, x;
  x = strlen(str);
  int x1 = 0, x2 = 0;
  if (n == 1) {
    cout << "NO" << endl;
    return 0;
  }
  if (n == 2) {
    if (str[0] == str[1]) {
      cout << "NO" << endl;
      return 0;
    } else {
      cout << "YES" << endl;
      cout << str << endl;
      return 0;
    }
  }
  for (i = 0; i < x - 1; i++) {
    if (str[i] != str[i + 1]) {
      cout << "YES" << endl;
      cout << str[i] << str[i + 1] << endl;
      return 0;
    }
  }
  cout << "NO" << endl;
  return 0;
}
