158B-Taxi

#include<bits/stdc++.h>
using namespace std;

int main()
{
    int one = 0, two =0, three = 0, four = 0;

    int  n; cin>>n;

    for(int i=0; i<n; i++)
    {
        int x; cin>>x;

        if(x==1)
            one++;
        if(x==2)
            two++;
        if(x==3)
            three++;
        if(x==4)
            four++;
    }

    int c = four;   // 4 person per taxi

    while(one!=0 && three!=0)   // for all 1 and 3 pairs
    {
        one--;
        three--;
        c++;
    }

    if(one==0 && three!=0)  // Adding remaining threes; Remaining ones will
        c = c + three;      // be taken care from Line 58;


    c = c + two/2;

    if(two%2!=0)    // Remaining 2 if exist
    {
        if(one<=2)  // if 1 or 2 one's left
            one = 0;
        else
            one = one - 2;

        two = 0;
        c++;
    }

    if(one!=0)      // all remaining ones
    {
        c = c + one/4;

        if(one%4!=0)
            c++;
    }

    cout << c;
}