CS50 Problem Set 1 (Fall 2019) - Credit

Credit.c

//CS50 Problem Set 1 (Fall 2019): Credit
//Author: teeschorle

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

void checksum(string card);
int digit(int position);

long number;
int digits;
    
int main(void)
{
    number = get_long("Number: ");
    {
        //count digits
        digits = trunc(log10(number)) + 1;  

        //case: AmEx
        if (digit(1) == 3 && (digit(2) == 4 || digit(2) == 7) && digits == 15)        
        {
            checksum("AMEX");
        }  
        
        //case: MasterCard
        else if (digit(1) == 5 && ((digit(2) < 6) || (digit(2) > 0)) && digits == 16)
        {
            checksum("MASTERCARD");
        }
        
        //case: Visa
        else if (digit(1) == 4 && (digits == 13 || digits == 16))
        {
            checksum("VISA");
        }
        
        //case: none of the above
        else
        {
            printf("INVALID\n");
        }
    }
}

//return digit at n-th position in card number
int digit(int n)
{ 
    return ((long)(number / pow(10, digits - n))) % 10;
}

//check sum
void checksum(string card)
{
    int sum = 0;
    //doubling every other digit and summing up results by digits
    for (int i = digits - 1; i > 0; i = i - 2)
    {
        if (digit(i) < 5)
        {
            sum = sum + 2 * digit(i);
        }
        else
        {
            sum = sum + 1 + ((2 * digit(i)) % 10);
        }
    }
    //adding those digits that haven't been doubled
    for (int i = digits; i > 0; i = i - 2)
    {
        sum = sum + digit(i);
    }
    //check if last digit of sum equals zero
    if (sum % 10 == 0)
    {
        printf("%s\n", card);
    }
    else
    {
        printf("INVALID\n");
    }
}

help me
#include <stdio.h>
#include <cs50.h>
void print_credit_card_brand(long long ccn);
bool check_validity(long long credit_card_number);
int find_length(long long n);
bool checksum(long long ccn);

int main(void){

long long credit_card_number;
do
{
credit_card_number= get_long_long("number: ");
}while(credit_card_number<0);

if((credit_card_number))
print_credit_card_brand(credit_card_number);
else
printf("INVALID\n");
}

bool check_validity(long long credit_card_number )
{
int len = find_length (credit_card_number);
return (len==13 || len==15 || len ==16) && checksum(credit_card_number);
}

int fine_length(long long n)
{
int len ;
for(len =0 ; n != 0; n /= 10 , len++);
return len;
}

bool checksum(long long ccn)
{
int sum=0;
for(int i=0 ; ccn != 0 ;i++ , ccn /=10)
{

if(i%2==0)
sum += ccn % 10;
else{
int digit= 2 * (ccn % 10);
sum += digit /10 + digit % 10;
}
}
return (sum %10)==0;
}

void print_credit_card_brand(long long ccn)
{
if((ccn>=34e13 && ccn<35e13) ||(ccn>=37e13 && ccn<38e13) )
printf("AMEX\n");
else if (ccn>=51e14 && ccn<56e14)
printf("MASTERCARD\n");
if((ccn>=4e12 && ccn<5e12) ||(ccn>=4e15 && ccn<5e15) )
printf("VISA\n");
else
printf("INVALID");
}