#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 6, nBits = 2e8 + 5, M = (1 << 16), MAX = 1111,
          OO = 0x3f3f3f3f, MOD = 1e9 + 7, inf = 1 << 30;
const long long INF = (long long)1e18;
long long GCD(long long a, long long b) { return !b ? a : GCD(b, a % b); }
long long LCM(long long x, long long y) { return (x * y / GCD(x, y)); }
long long fact(long long z) { return (z <= 1) ? 1 : z * fact(z - 1); }
int n, freq[N];
string s, ss;
bool f;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> s;
  for (int i = 1; i < n; ++i)
    if (s[i] != s[i - 1]) {
      ss += s[i - 1], ss += s[i];
      f = 1;
      break;
    }
  if (f)
    cout << "YES\n" << ss;
  else
    cout << "NO\n";
  return 0;
}
