Grading statistics

Write a program that reads the grades of the students of a certain subject until the user enters the word “exit”. The exit of your program must write at the end the number of passed, the number of failed, and the average grade. Remember that a string can be converted to a float, by calling float. Test it:

>>> s = "3.456"
>>> float(s)
3.456
>>> s = "-3.456"
>>> float(s)
-3.456

Some examples of program executions are below. There you can see how your program should handle negative numbers.

>>> %Run 
    Enter a grade or 'exit': 3.5
    Enter a grade or 'exit': 0
    Enter a grade or 'exit': 10
    Enter a grade or 'exit': 9.99
    Enter a grade or 'exit': 5
    Enter a grade or 'exit': 6
    Enter a grade or 'exit': 8
    Enter a grade or 'exit': exit
    Passed: 5
    Failed: 2
    Average grade: 6.07
>>> %Run 
    Enter a grade or 'exit': -4
    You cannot enter negative amounts.
    Enter a grade or 'exit': 4
    Enter a grade or 'exit': -6.4
    You cannot enter negative amounts.
    Enter a grade or 'exit': 0
    Enter a grade or 'exit': 3
    Enter a grade or 'exit': exit
    Passed: 0
    Failed: 3
    Average grade: 2.3333333333333335  

Insist that the students test their programs by giving them example
test executions.