#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 5;
const int INF = 0x3f3f3f3f;
long long n, sum, tmp, T, f;
long long a[maxn], num[maxn];
string s, t;
int main() {
  cin >> n >> s;
  for (int i = 1; i < n; i++) {
    if (s[i - 1] != s[i]) {
      cout << "YES" << endl;
      cout << s[i - 1] << s[i] << endl;
      return 0;
    }
  }
  cout << "NO" << endl;
  return 0;
}
