input: Write a function to compute the sum of two variables and store the result in a third variable. If the given variables are not integers, return None.
  assert compute_sum(5, 8)==13
  assert compute_sum("a", 8)==None
  assert compute_sum(3.5, 4.5)==8.0
  assert compute_sum(10, -5)==5
  assert compute_sum(0, 0)==0
output: def compute_sum(a, b):
      if isinstance(a, int) and isinstance(b, int):
          return a + b
      return None