#include <iostream>
using namespace std;
int main ()
{
    int n,a,a1,a2,a3,temp;
    cin>>n;
    temp=0;
	a1=0;
	a2=0;
	a3=0;
	for (int i=0;i<n;i++)
	{
        cin>>a;
        if (a==1)
        a1++;
        else if (a==2)
        a2++;
        else if (a==3)
        a3++;
        else if (a==4)
        temp++;
    }
    temp+=a3;
    a1-=a3;
    temp+=a2/2;
    if (a2%2!=0)
    {
       a1-=2;
       temp+=1;
    }
    if (a1<0)
    a1=0;
    temp+=(a1+3)/4;
    cout<<temp<<endl;
}