#include <bits/stdc++.h>
using namespace std;
string t = "abcdefghijklmnopqrstuvwxyz";
int k[30], n;
string s, m;
bool kt(string a) {
  int n = a.length();
  for (int j = 0; j < t.length(); j++) {
    k[j] = 0;
    for (int i = 0; i < a.length(); i++) {
      if (a[i] == t[j]) k[j]++;
    }
  }
  for (int j = 0; j < t.length(); j++) {
    if (k[j] > (n / 2)) return 0;
  }
  return 1;
}
void nhap() {
  cin >> n;
  cin >> s;
}
void xuli() {
  int dem = 0;
  for (int i = 0; i < s.length() - 1; i++) {
    if (s[i] != s[i + 1]) {
      dem++;
      m = s.substr(i, 2);
    }
  }
  if (dem == 0) cout << "NO";
  if (dem > 0) cout << "YES" << endl << m;
}
int main() {
  nhap();
  xuli();
}
