(CS50) Credit

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

int main(void)
{
    long card, cardcheck;
    int cardlen = 0, even = 0, odd = 0, sumeo = 0;

    do
    {
        card = get_long("Introduce the card: ");
    }
    while (card < 0);

    for (long i = card; i != 0; i = i / 10, cardlen++); // looking card lengh

    if (cardlen < 13 || cardlen == 14 || cardlen > 16) // is a card?
    {
        printf("INVALID\n");
        return 0; //if not a card close app
    }
    //checksum
    cardcheck = card;
    while (cardcheck  > 0)
    {
        int n1 = cardcheck % 10;
        even += n1;
        cardcheck /= 10;

        int n2 = cardcheck % 10;
        n2 *= 2;
        if (n2 > 9)
        {
            int a = n2 % 10;
            int b = n2 / 10;
            n2 = a + b;
        }
        odd += n2;
        cardcheck /= 10;
    }

    sumeo = even + odd;
    if (sumeo % 10 == 0)
    {
        if ((card >= 340000000000000 && card < 350000000000000) || (card >= 370000000000000 && card < 380000000000000))
        {
            printf("AMEX\n");
        }
        else if (card >= 5100000000000000 && card < 5600000000000000)
        {
            printf("MASTERCARD\n");
        }
        else if ((card >= 4000000000000000 && card < 5000000000000000) ||(card >=  4000000000000 && card < 5000000000000))
        {
            printf("VISA\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }
}