cs50 credit

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

int main(void)
{
    long number = get_long("Number: ");

    //if(number is less than 15 or greater than 16)

    //find how many digits number is
    int digits = 0;
    long copy = number;
    int first_digit = 0;
    int first_2_digits = 0;
    while (copy > 0)
    {
        copy = copy / 10;
        digits += 1;
        if (copy < 100 && copy > 10)
        {
            first_2_digits = copy;
        }
        if (copy < 10 && copy > 0)
        {
            first_digit = copy;
        }
    }

    if (digits == 13 || digits == 15 || digits == 16)
    {
        copy = number * 10;
        int checksum = 0;
        //get the sum of every other number * 2
        while(copy > 0)
        {
            copy = copy / 100;
            int answer = (copy % 10) * 2;
            if (answer > 9)
            {
                answer = answer % 10 + answer / 10;
            }
            checksum += answer;
        }

        copy = number;
        while (copy > 0)
        {
            checksum += copy % 10;
            copy = copy / 100;

        }

        if (checksum % 10 == 0)
        {
            if (digits == 15 && (first_2_digits == 34 || first_2_digits == 37))
            {
                printf("AMEX\n");
            }
            else if (digits == 13 && first_digit == 4)
            {
                printf("VISA\n");
            }
            else
            {
                if (first_digit == 4)
                {
                    printf("VISA\n");
                }
                else if (first_2_digits == 51 || first_2_digits == 52 || first_2_digits == 53 || first_2_digits == 54 || first_2_digits == 55)
                {
                    printf("MASTERCARD\n");
                }
                else
                {
                    printf("INVALID\n");
                }
            }
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }

    //calculate checksum
    //check card length and starting digits
    //print AMEX, MASTERCARD, VISA or INVALID

}