Problem 64. The Goldbach Conjecture, Part 2

The Goldbach conjecture asserts that every even integer greater than 2 can be expressed as the sum of two primes.

Given the even integer n, return c, the number of different ways two primes can be added to result in n. Only count a pair once; the order is unimportant.

Example:

 Input  n = 10
 Output c is 2

because of the prime pairs [3 7] and [5 5].

 Input  n = 50
 Output c is 4

because of [3 47], [7 43], [13 37], and [19 31].