credit

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

int company0(long long Number);
int company1(long long Number);
int summation(long long Number, int digit);



int main(void)
{
    //getting intial valid number from the user
long long Number;
do{
    Number = get_long_long("Number: ");

}while(Number < 400000000 || Number > 10000000000000000);//it was like this before but you didn't like (Number < 4000000000000 || Number > 10000000000000000);

int LN = company0(Number);
int sum = company1(Number);
    if((LN == 34 || LN == 37) && (sum % 10 == 0))
    {
        printf("AMEX\n");
    }
    else if((LN > 50 && LN < 56) && (sum % 10 == 0))
    {
        printf("MASTERCARD\n");
    }
    else if((LN > 39 && LN < 50) && (sum % 10 == 0))
    {
        printf("VISA\n");
    }
    else
    {
        printf("INVALID\n");
    }
}

int summation(long long Number, int digit) //The summation function which depends on the other 2 functions below
{
    int i, sum = 0, xsum =0, x;
    for(i = 0; i < digit; i++)
    {
        if(i%2 == 0)
        {
           x = Number % 10;
           sum += x;
           Number /= 10;
        }
        else
        {
            x = Number % 10;
            x *= 2;
            if(x < 10)
            {
                xsum += x;
                Number /= 10;
            }
            else
            {
                x %= 10;
                xsum += x + 1;
                Number /= 10;
            }
        }
    }
    return sum + xsum;
}

int company1(long long Number) //calculate the number of the digits in the credit card number to use it later and call another function for that
{
    int digit = 1;
    long long original = Number;
    while(Number > 10)
    {
        Number /= 10;
        digit++;
    }
    if(digit < 13)
    {
        printf("INVALID\n");
        return 0;
    }
   int sum = summation(original, digit);
   return sum;
}



int company0(long long Number) //check initially if it's fit any of the companies
{
int lastNumber;
while(Number > 100)
{
    Number /= 10;
}
lastNumber = Number;
    return lastNumber;
}