#include <bits/stdc++.h>

using namespace std;

int main() {
  int N;
  cin >> N;
  string S;
  cin >> S;
  int x = 0, y = 0, z = 0, ret = 0;

  int one_stock = 0;
  for(auto &c : S) {
    if(c == '3' || c == '5' || c == '7') {
      ++ret;
    } else if(c == '1') {
      ++x;
      ++one_stock;
    } else {
      ++y;
      if(one_stock > 0) { // 19
        --one_stock;
        ++z;
      }
    }
  }
  int v = min((y - z) / 2, x - z); // 991
  cout << ret + z + v + (x - z - v) / 2 << endl;
}