#include <stdio.h>

int main ()
{
   char cardNo[17];
   int sum;

   printf("Enter a 16-digit credit card number : "); scanf("%s",cardNo);
   sum  = (cardNo[1] - '0') + (cardNo[3] - '0') + (cardNo[5] - '0') +
          (cardNo[7] - '0') + (cardNo[9] - '0') + (cardNo[11] - '0') +
          (cardNo[13] - '0') + (cardNo[15] - '0');
   sum += 2 * (int)((cardNo[0] - '0') + (cardNo[2] - '0') +
          (cardNo[4] - '0') + (cardNo[6] - '0') + (cardNo[8] - '0') +
          (cardNo[10] - '0') + (cardNo[12] - '0') + (cardNo[14] - '0'));
   if (cardNo[0]  >= '5') sum -= 9;
   if (cardNo[2]  >= '5') sum -= 9;
   if (cardNo[4]  >= '5') sum -= 9;
   if (cardNo[6]  >= '5') sum -= 9;
   if (cardNo[8]  >= '5') sum -= 9;
   if (cardNo[10] >= '5') sum -= 9;
   if (cardNo[12] >= '5') sum -= 9;
   if (cardNo[14] >= '5') sum -= 9;
   printf("Checksum = %d\n", sum);
   if (sum % 10 == 0) printf("This is a valid credit card number.\n");
   else printf("This is an invalid credit card number.\n");
}