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

int main(){
    int n;
    cin >> n;
    string s;
    cin >> s;
    vector<int> one, nine;
    int ans = 0;
    for(int i = 0; i < n; i++){
        if(s[i] == '3' || s[i] == '5' || s[i] == '7') ans++;
        else if(s[i] == '1') one.push_back(i);
        else{
            if(one.size()) one.pop_back(), ans++;
            else nine.push_back(i);
        }
    }
    int tmp = min(nine.size() / 2, one.size());
    ans += tmp;
    ans += (one.size() - tmp) / 2;
    cout << ans << endl;
}