#include <bits/stdc++.h>
using namespace std;

int main(){
    int n;
    cin >> n;
    string s;
    cin >> s;
    string t = "";
    int ans = 0;
    for(int i = 0; i < n; i++){
        if(s[i] == '3' || s[i] == '5' || s[i] == '7') ans++;
        else t += s[i];
    }
    int m = t.size();
    int preone = 0;
    int prenine = 0;
    for(int i = 0; i < m; i++){
        if(t[i] == '1'){
            if(prenine <= 1){
                preone++;
            }else{
                prenine -= 2;
                ans++;
            }
        }else{
            if(preone != 0){
                preone--;
                ans++;
            }else{
                prenine++;
            }
        }
    }
    ans += preone / 2;
    cout << ans << endl;
}