#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios::sync_with_stdio(false); cin.tie(0);
  int n;
  cin >> n;
  string s;
  cin >> s;
  int ans = 0;
  int one = 0, nine = 0;
  for (char c : s) {
    if (c == '3' || c == '5' || c == '7') ans++;
    else {
      if (c == '1') {
        one++;
      }
      if (c == '9') {
        if (one > 0) {
          one--;
          ans++;
        } else {
          nine++;
        }
      }
    }
  }
  ans += min(one, nine / 2);
  one -= min(one, nine / 2);
  ans += one / 2;
  cout << ans << '\n';
  return 0;
}