#include <bits/stdc++.h>
using namespace std;
long long int power(long long int x, long long int y) {
  long long int r = 1;
  while (y > 0) {
    if (y % 2 != 0) r = r * x;
    x = x * x;
    y = y / 2;
  }
  return r;
}
long long int ncr(long long int n, long long int k) {
  long long int c[k + 1];
  memset(c, 0, sizeof(c));
  c[0] = 1;
  for (long long int i = 1; i <= n; i++) {
    for (long long int j = min(i, k); j > 0; j--) c[j] = c[j] + c[j - 1];
  }
  return c[k];
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long int n, i, j, p, f = 0;
  cin >> n;
  string s;
  cin >> s;
  if (n == 1) {
    cout << "NO";
    return 0;
  }
  for (i = 1; i < n; i++) {
    if (s[i] != s[i - 1]) {
      f = 1;
      p = i;
      break;
    }
  }
  if (f == 0)
    cout << "NO";
  else {
    cout << "YES\n";
    cout << s[p - 1] << s[p];
  }
  return 0;
}
