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

int main(){
    int n;
    string s;
    cin >> n >> s;
    assert(1 <= n && n <= 500000);
    int one = 0, nine = 0, ans = 0;
    for(int i = 0; i < n; i++){
        assert(s[i] == '1' || s[i] == '3' || s[i] == '5' || s[i] == '7' || s[i] == '9');
        if(s[i] == '3' || s[i] == '5' || s[i] == '7') ans++;
        else if(s[i] == '1'){
            if(nine >= 2) nine -= 2, ans++;
            else one++;
        }else{
            if(one >= 1) one--, ans++;
            else nine++;
        }
    }
    ans += one / 2;
    cout << ans << endl;
}