Average of Test Scores

Write a program to take user's test scores as many as he/she needs.
The computer will calculate the average and display the result with data

Steps:

1. Display the question as below
   
2. User may enter letters or numbers not in a range 0 to 100, or not -1, as below:
   
   
   You give them an alert, like
   
3. Calculate the average and display the results as below:
   Note: the number of test scores is not fixed.