#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  string x, s;
  int n;
  cin >> n;
  cin >> x;
  vector<char> lol;
  for (int i = 0; i < n; i++) {
    if (lol.size() == 2) {
      lol.pop_back();
    }
    lol.push_back(x[i]);
    if (lol.size() == 2) {
      if (lol[0] != lol[1]) {
        cout << "YES" << '\n';
        cout << lol[0] << lol[1] << '\n';
        return 0;
      }
    }
  }
  cout << "NO" << '\n';
  return 0;
}
