figuring out how to find the average of entered numbers

figuring out how to find the average of entered numbers

I need help figuring out how to find the average of entered numbers with
this code that i wrote:
sum = 0.0
average = number / sum
data = input("Enter a number or enter 999 to quit: ")
while data != "999":
number = float(data)
sum += number
data = input("Enter a number or enter 999 to quit: ")
print("The sum is", average)