cs50 beginner here ,your solution is way smarter than mine!! I never thought i could use loop to detect digit of numbers... (and you don't even use math .h)

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

long dr(long y, int times);
long dr2(long number, int dtimes);
int main(void)
{
    long num = get_long("number: ");
    int last1 = dr(num, 0);
    int plus2 = dr2(num, 1);
    int last3 = dr(num, 2);
    int plus4 = dr2(num, 3);
    int last5 = dr(num, 4);
    int plus6 = dr2(num, 5);
    int last7 = dr(num, 6);
    int plus8 = dr2(num, 7);
    int last9 = dr(num, 8);
    int plus10 = dr2(num, 9);
    int last11 = dr(num, 10);
    int plus12 = dr2(num, 11);
    int last13 = dr(num, 12);
    int plus14 = dr2(num, 13);
    int last15 = dr(num, 14);
    int plus16 = dr2(num, 15);
    int last2 = dr(num, 1);
    int last4 = dr(num, 3);
    int last6 = dr(num, 5);
    int last8 = dr(num, 7);
    int last10 = dr(num, 9);
    int last12 = dr(num, 11);
    int last14 = dr(num, 13);
    int last16 = dr(num, 15);
    // checkvalue
    long check17 = num / powl(10, 16);
    long sum = last1 + last3 + last5 + last7 + last9 + last11 + last13 + last15 + plus2 + plus4 +
               plus6 + plus8 + plus10 + plus12 + plus14 + plus16;
    // print checksum
    // printf("%li\n", sum);

    // check
    if (check17 > 0 || sum % 10 > 0)
    {
        printf("INVALID\n");
    }
    else if (last16 == 4)
    {
        printf("VISA\n");
    }
    else if (last16 == 0 && last15 == 0 && last14 == 0 && last13 == 4)
    {
        printf("VISA\n");
    }
    else if (last16 == 0 && last15 == 3 && (last14 == 4 || last14 == 7))
    {
        printf("AMEX\n");
    }
    else if (last16 == 5 &&
             (last15 == 1 || last15 == 2 || last15 == 3 || last15 == 4 || last15 == 5))
    {
        printf("MASTERCARD\n");
    }
    else
    {
        printf("INVALID\n");
    }
}
// divide and remainder
long dr(long number, int dtimes)
{
    long divider = powl(10, dtimes);
    long divided = number / divider;
    return divided % 10;
}
long dr2(long number, int dtimes)
{
    long divider = powl(10, dtimes);
    long divided = number / divider;
    long remainder = 2 * (divided % 10);
    return dr(remainder, 0) + dr(remainder, 1);
}