My solution to CS50 2022 psets-2 credit

credit.c

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

//AMERX 15 STRT 34 OR 37
//MC 16 STRT 51, 52, 53, 54, 55
//VZA 13 OR 16 STRT 4

int main(void)
{
    long creditNumber;

    do
    {
        creditNumber = get_long("Number: ");
    }
    while (creditNumber <= 0);

    long creditCard = creditNumber;
    int sum = 0;
    int count =0;
    long divider = 10;

    while (creditCard > 0)
    {
        int lastDigit = creditCard % 10;
        sum = sum + lastDigit;
        creditCard = creditCard / 100;
    }

    creditCard = creditNumber / 10;
    while (creditCard > 0)
    {
        int lastDigit = creditCard % 10;
        int byTwo = lastDigit * 2;
        sum = sum + (byTwo % 10) + (byTwo / 10);
        creditCard = creditCard / 100;
    }

    creditCard = creditNumber;
    while (creditCard != 0)
    {
        creditCard = creditCard / 10;
        count++;
    }

    for (int i = 0; i < count - 2; i++)
    {
        divider = divider * 10;
    }

    int firstDigit = creditNumber / divider;
    int firstTwoDigits = creditNumber / (divider / 10);

    if ((sum % 10) == 0)
    {
        if (firstDigit == 4 && (count == 13 || count == 16))
        {
            printf("VISA\n");
        }
        else if ((firstTwoDigits == 34 || firstTwoDigits == 37) && count == 15)
        {
            printf("AMEX\n");
        }
        else if ((firstTwoDigits > 50 && firstTwoDigits < 56) && count == 16)
        {
            printf("MASTERCARD\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else {
        printf("INVALID\n");
    }
}