Credit CS50 PSET1

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    //Getting Card Number
    long num = get_long("Number: ");

    //Length Count

    long dum = num;
    int digit = 0;
    while (dum>0)
    {
        dum = dum/10;
        digit++;
    }

    //Check Length

    if(digit != 13 && digit != 15 && digit != 16)
    {
        printf("INVALID\n");
        return 0;
    }



    long dum2 = num;
    int sumodd = 0;
    int sumeven = 0;
    int rem1 = 0;
    int rem2 = 0;
    int digit1 = 0;
    int digit2 = 0;

    do
    {   //Remove the Last Digit
        rem1 = dum2%10;
        sumodd += rem1;
        dum2 = dum2/10;

        //Remove 2nd Last Digit
        rem2 = dum2%10;
        dum2 = dum2/10;

        //Double the Digit and Add Sum of Digits
        rem2 = rem2*2;
        digit1 = rem2%10;
        rem2 = rem2/10;
        sumeven = sumeven + digit1 + digit2;
    }
    while (dum2>0);

    int final = sumeven + sumodd;

    //Checking if final is a multiple of 10

    long dum3 = num;

    if (final%10 == 0)
    {
        do
        {
            dum3 = dum3/10;
        }
        while(dum>0);

        if((dum3 / 10 == 5) && (0 < dum3 % 10 && dum3 % 10 < 6))
        {
            printf("MASTERCARD\n");

        }
        else if (dum3 == 34 || dum3 == 37)
        {
            printf("AMEX\n");
        }
        else if ((dum3/10) == 4)
        {
            printf("VISA\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
        return 0;
    }
}