input: Write a function to add two numbers taken as input from the user.
  assert add_two_numbers()==(result of adding two input numbers)
  assert add_two_numbers()==(result of adding two input numbers)
  assert add_two_numbers()==(result of adding two input numbers)
output: def add_two_numbers():
      num1 = float(input("Enter first number: "))
      num2 = float(input("Enter second number: "))
      return num1 + num2