Add number of squares constant in grains

For clarity.

c/grains/src/grains.c

@@ -1,11 +1,13 @@
 #include "grains.h"
 
+const int NUMBER_OF_SQUARES = 64;
+
 // square returns the number of grains to be placed at the nth square.
 // An unsigned long long is necessary to hold the value of 2^64, which
 // on 64-bit is equivalent to the max value of that type (ULLONG_MAX).
 // square returns 0 for non-existing squares.
 unsigned long long square(int n) {
-  if (n < 1 || n > 64) return 0;
+  if (n < 1 || n > NUMBER_OF_SQUARES) return 0;
   // A left shift of 0 leaves a number unchanged.
   // Otherwise, each left shift of 1 doubles the number;
   // i.e, 1 << 0 = 1, 1 << 1 = 2, 1 << 2 = 4, etc.
@@ -18,7 +20,7 @@ unsigned long long square(int n) {
 unsigned long long total() {
   unsigned long long n = 0;
   // Add up all the squares.
-  for (int i = 1; i <= 64; i++) {
+  for (int i = 1; i <= NUMBER_OF_SQUARES; i++) {
     n += square(i);
   }
   return n;