input: Write a function to compute the sum of two variables and store the result in a third variable. Then, print the result. If the given input is invalid, return None.
  assert compute_sum(5, 8)==13
  assert compute_sum(0, 0)==0
  assert compute_sum(10, -10)==0
  assert compute_sum(None, 5)==None
  assert compute_sum(5, None)==None
output: def compute_sum(a, b):
      if a is None or b is None:
          return None
      result = a + b
      return result