#include <bits/stdc++.h>

using namespace std;

int main() {
  int N;
  cin >> N;
  string S;
  cin >> S;
  int ret = 0;
  string T;
  for(auto &c : S) {
    if(c == '3' || c == '5' || c == '7') {
      ++ret;
    } else {
      T += c;
    }
  }

  vector< int > used(T.size());
  vector< int > nine, one;


  for(int i = 0; i < T.size(); i++) {
    if(used[i]) continue;
    if(T[i] == '1') {
      one.emplace_back(i);
    } else if(one.size()) {
      used[one.back()] = true;
      one.pop_back();
      used[i] = true;
      ++ret;
    }
  }

  for(int i = 0; i < T.size(); i++) {
    if(used[i]) continue;
    if(T[i] == '9') {
      nine.emplace_back(i);
    } else if(nine.size() >= 2) {
      used[nine.back()] = true;
      nine.pop_back();
      used[nine.back()] = true;
      nine.pop_back();
      used[i] = true;
      ++ret;
    }
  }

  int a = 0;
  for(int i = 0; i < T.size(); i++) {
    if(used[i]) continue;
    if(T[i] == '1') ++a;
  }

  cout << ret + a / 2 << endl;
}