input: Write a function to return the sum of two parameters. The function should be named add and it should take two parameters x and y. If the parameters are numerical values, it should return their sum. If either parameter is not a number, it should return a message indicating that the input is invalid.
  assert add(3, 4)==7
  assert add(0, 0)==0
  assert add(-3, 3)==0
  assert add("a", 4)=="Invalid input: both parameters should be numerical"
  assert add(5, "b")=="Invalid input: both parameters should be numerical"
output: def add(x, y):
      if isinstance(x, (int, float)) and isinstance(y, (int, float)):
          return x + y
      else:
          return "Invalid input: both parameters should be numerical"