// ???
#include<iostream>
using namespace std;

int main(){
    int n;
    string s;
    cin >> n >> s;
    int ans = 0, one = 0, five = 0;
    for(int i = 0; i < n; i++){
        if(s[i] == '3' || s[i] == '7'){
            ans++;
            continue;
        }
        if(s[i] == '9'){
            if(five > 0){
                ans++;
                five--;
            }else if(one > 0){
                ans++;
                one--;
            }
        }else if(s[i] == '1'){
            one++;
        }else if(s[i] == '5'){
            five++;
        }
    }
    cout << ans+one/2 << endl;
    return 0;
}