#include <iostream>
using namespace std;
string z;
int main(){
  int n;
  string s;cin>>n>>s;
  int ans = 0;
  for(int i = 0; n > i; i++){
    if(s[i] == '3' || s[i] == '5' || s[i] == '7')ans++;
    else z.push_back(s[i]);
  }
  int tmp = 0;
  for(int i = z.size()-1;0 <= i; i--){
    if(z[i] == '1'){
      if(tmp){
        ans++;
        tmp--;
      }else{
        tmp++;
      }
    }else{
      tmp++;
    }
  }
  cout << ans << endl;
}