#include <iostream>

using namespace std;

int n, z[5];

int main(){
	cin >> n;
	int x;
	for (int i = 1;i <= n;i ++){
		cin >> x;
		z[x]++;
	}
	int res=z[4];
	if (z[3]<z[1]){
		res+=z[3];
		z[1] = z[1]-z[3];
	}else{
		res+=z[3];
		z[1] = 0;
	}
	res+=z[2]/2;
	res+=z[1]/4;
	if (z[2]>0) z[2] = z[2]%2;
	if (z[1]>0) z[1] = z[1]%4;
	if (z[2]>0){
		if (z[1]==3) res+=2;
		else res+=1;
	}else{
		if (z[1]>0) res+=1;
	}
	cout << res;
	return 0;
	
}