#include <bits/stdc++.h>

using namespace std;

int main() {
  int N;
  cin >> N;
  string S;
  cin >> S;
  int one = 0, nine = 0, ret = 0;
  for(auto &c : S) {
    if(c == '3' || c == '5' || c == '7') {
      ++ret;
    } else if(c == '1') {
      ++one;
    } else {
      if(one > 0) --one, ++ret;
      else ++nine;
    }
  }
  int v = min(nine / 2, one);
  one -= v;
  cout << ret + v + one / 2 << endl;
}