input: Write a python function to calculate the sum of all even numbers from 2 to n inclusive.
  assert sum_even_numbers(10) == 30
  assert sum_even_numbers(20) == 110
  assert sum_even_numbers(50) == 650
output: def sum_even_numbers(n):
      return sum(i for i in range(2, n+1) if i % 2 == 0)