#include <bits/stdc++.h>
using namespace std;
const int INF32M = 0x3f3f3f3f;
const long long INF64M = 0x3f3f3f3f3f3f3f3f;
int main() {
  ios::sync_with_stdio(false);
  int n, i, flag = 0;
  cin >> n;
  string s;
  cin >> s;
  int m = s.size();
  for (i = 0; i < m - 1; i++)
    if (s[i] != s[i + 1]) {
      flag = 1;
      break;
    }
  if (flag)
    cout << "YES" << endl << s[i] << s[i + 1] << endl;
  else
    cout << "NO" << endl;
  return 0;
}
