#include <bits/stdc++.h>

using namespace std;

int N;
string s;
int ans, one, nine;

int main() {
  cin >> N >> s;

  for (int i = 0; i < N; i++) {
    if (s[i] == '3' || s[i] == '7' || s[i] == '5') ans++;
    if (s[i] == '1') {
      one++;
    }
    if (s[i] == '9') {
      if (0 < one) {
        ans++;
        one--;
      } else {
        nine++;
      }
    }
  }
  ans += one / 2;
  cout << ans << endl;

  return 0;
}