#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i, j, mark = 0;
  cin >> n;
  char input[1005], output[1005];
  cin.ignore();
  cin >> input;
  for (i = 1; i < n; i++) {
    if (input[i] == input[i - 1])
      continue;
    else {
      output[0] = input[i - 1];
      output[1] = input[i];
      output[2] = '\0';
      cout << "YES" << endl;
      printf("%s", output);
      mark = 1;
      return 0;
    }
  }
  if (mark == 0) cout << "NO";
  return 0;
}
