input: Write a function to calculate the average of five numbers entered by the user. The numbers should be inputted one by one and stored in a list. Once all five numbers are entered, the function should calculate and return the average of these numbers. If no numbers are entered, the function should return 0.
  assert average_of_five(1, 2, 3, 4, 5)==3.0
  assert average_of_five(10, 20, 30, 40, 50)==30.0
  assert average_of_five(0, 0, 0, 0, 0)==0.0
  assert average_of_five(-1, -1, -1, -1, -1)==-1.0
  assert average_of_five()==0.0
output: def average_of_five(a=0, b=0, c=0, d=0, e=0):
      numbers = [a, b, c, d, e]
      if all(num == 0 for num in numbers):
          return 0
      return sum(numbers) / 5