#include <vector>
#include <algorithm>

bool exists(std::vector<int>, int);
using namespace std;

int main() {
    int n;
    cin >> n;
    vector<int> vec;
    for (int i = 0; i < n; i++) {
        int temp;
        cin >> temp;
        vec.push_back(temp);
    }

    sort(vec.begin(), vec.end());

    int c = 0;
    while (vec.size() > 0) {
        for (int i = 0; i < vec.size(); i++) {
            if (vec[i] == 1) {
                if (exists(vec, 3)) {
                    vec.erase(vec.begin() + i);
                    vec.erase(find(vec.begin(), vec.end(), 3));
                    c++;
                }
                else if (exists(vec, 2)) {
                    if (exists(vec, 1) && count(vec.begin(), vec.end(), 1) > 1) {
                        vec.erase(find(vec.begin(), vec.end(), 1));
                        vec.erase(find(vec.begin() + i + 1, vec.end(), 1));
                        vec.erase(find(vec.begin(), vec.end(), 2));
                        c++;
                    }
                    else {
                        vec.erase(find(vec.begin(), vec.end(), 1));
                        vec.erase(find(vec.begin(), vec.end(), 2));
                        c++;
                    }
                }
                else if (count(vec.begin(), vec.end(), 1) > 1) {
                    if (count(vec.begin(), vec.end(), 1) >= 4)
                    {
                        for (int j = 0; j < 4; j++) {
                            vec.erase(find(vec.begin(), vec.end(), 1));
                        }
                    }
                    else {
                        for (int j = 0; j < count(vec.begin(), vec.end(), 1); j++) {
                            vec.erase(find(vec.begin(), vec.end(), 1));
                        }
                    }
                    cout << endl;
                    c++;
                }
            }
            else if (vec[i] == 2) {
                if (exists(vec, 2) && count(vec.begin(), vec.end(), 2) > 1) {
                    vec.erase(find(vec.begin(), vec.end(), 2));
                    vec.erase(find(vec.begin(), vec.end(), 2));
                    c++;
                }
                else {
                    vec.erase(find(vec.begin(), vec.end(), 2));
                    c++;
                }
            }
            else if (vec[i] == 3) {
                vec.erase(find(vec.begin(), vec.end(), 3));
                c++;
            }
            else {
                vec.erase(find(vec.begin(), vec.end(), 4));
                c++;
            }
        }
    }
    cout << c;
    return 0;

}

bool exists(vector<int> v, int x) {
    if (find(v.begin(), v.end(), x) != v.end())
        return true;
    else
        return false;
}