B. Taxi Solution

#include<bits/stdc++.h>

using namespace std;

int main()

{

    int n,a,total=0,m1=0,m2=0,m3=0;

    cin>>n;

    while(n--){

        cin>>a;

        if(a==4)

            total++;

        else if (a==3)

            m3++;

        else if (a==2)

            m2++;

        else if (a==1)

            m1++;

    }

    total += m2/2;

    total+=m2%2;

    if (m3>=m1){

        total += m1;

        total += m3-m1;

    }

    else if (m3<m1){

        total+= m3;

        m1 = m1-m3;

         if(m2%2==1 && m1>2){

            m1=m1-2;

            total += (m1/4);

            if (m1%4 != 0)

                total++;

        }

        else if (m2%2== 0){

            total += (m1/4);

            if (m1%4 != 0)

                total++;

        }

    }

    cout<<total<<endl;

    return 0;

}