#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (int)n; i++)
using namespace std;
using ll = long long;

int main() {
    int n;
    string s;
    cin >> n >> s;
    int ans = 0;
    string t = "";
    rep(i, n) {
        if(s[i] == '3' || s[i] == '5' || s[i] == '7') {
            ans++;
        } else {
            t += s[i];
        }
    }
    // 11, 19, 991
    int c1 = 0, c9 = 0;
    rep(i, t.size()) {
        if(t[i] == '1') {
            c1++;
        } else {
            if(c1) {
                c1--;
                ans++;
            } else c9++;
        }
    }
    while(c9 >= 2) {
        if(c1) {
            c9 -= 2;
            c1--;
            ans++;
        } else break;
    }
    while(c1 >= 2) {
        c1 -= 2;
        ans++;
    }
    cout << ans << endl;
    return 0;
}