input: Write a python function to count the number of pairs with even sum in a list.
  assert count_even_pairs(6, [1, 2, 3, 4, 5, 6]) == 6
  assert count_even_pairs(5, [1, 3, 5, 7, 9]) == 0
  assert count_even_pairs(4, [2, 4, 6, 8]) == 6
output: def count_even_pairs(n, lst):
      count = 0
      for i in range(n):
          for j in range(i + 1, n):
              if (lst[i] + lst[j]) % 2 == 0:
                  count += 1
      return count