Codeforces_158B

//Codeforces Round 158B
#include <cmath>
#include <iostream>
using namespace std;
int del(int a, int b) {
    return a/b + (a%b>0);
}
int main () {
    int n, k, ans, mas[5] = {};
    cin >> n;
    for (int i=0; i<n; i++) {
        cin >> k;
        mas[k] ++;
    }
    ans = mas[4];
    if (mas[3] <= mas[1]) {
        ans += mas[3];
        mas[1] -= mas[3];
    }
    else {
        ans += mas[3];
        mas[1] = 0;
    }
    mas[2] += del(mas[1], 2);
    ans += del(mas[2], 2);
    cout << ans;
    return 0;
}