A solution for Harvard / edX CS50 week 1 luhn algorithm credit card verification assignment

credit.c

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

int main(void) {
    long card_no;
    do{
        card_no = get_long("Card Number: ");
    }while(card_no < 1);

    long temp = card_no;
    int firstDigit, secondDigit;
    int sum = 0;
    int current;
    int counter = 0;
    int product;

    while(temp > 0) {
        counter++;
        current = temp % 10;
        if(counter % 2 == 0) { 
          product = current * 2;
          if(product > 9) {
              sum += floor(product / 10) + (product % 10);
          }else {
              sum += product;
          }
        }else {
             sum += current;
        }
        if(temp > 9) {
            secondDigit = current;
        }else if(temp <= 9) {
            firstDigit = current;
        }
        temp = temp /10;
    }

    if(sum % 10 == 0) {
        switch(firstDigit) {
            case 3:
                if( (counter == 15) && (secondDigit == 4 || secondDigit == 7  )) 
                {
                    printf("AMEX\n");

                }else {
                     printf("INVALID\n");
                }
                break;
            case 4:
                if((counter == 13 || counter == 16)) 
                {
                    printf("VISA\n");
                }else {
                     printf("INVALID\n");
                }
                break;
            case 5:
                if(counter == 16 && (secondDigit == 1 || secondDigit == 2 || secondDigit == 3 || secondDigit == 4 || secondDigit == 5  )) 
                {
                    printf("MASTERCARD\n");
                }else {
                     printf("INVALID\n");
                }
                break;
             default:
                printf("INVALID\n");
        }
    }else {
        printf("INVALID\n");
    }

}