input: Write a function to calculate the area of a rhombus.
  assert calculate_rhombus_area(10, 20) == 100
  assert calculate_rhombus_area(10, 5) == 25
  assert calculate_rhombus_area(4, 2) == 4
output: def calculate_rhombus_area(d1, d2):
      return d1 * d2 / 2