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

int main(){
    int n, ans = 0;
    string s;
    cin >> n >> s;
    int cnt1 = 0, cnt9 = 0;
    for(int i=0;i<n;i++){
        if(s[i] == '3' || s[i] == '5' || s[i] == '7') ans++;
        else if(s[i] == '1') cnt1++;
        else{
            // 19
            if(cnt1 > 0){
                ans++;
                cnt1--;
            }
        }
    }
    // 11
    ans += cnt1/2;
    cout << ans << endl;
    return 0;
}