"""
problem4.py
====

Author:  



Write a program that asks the user to continually enter student grades. 
To indicate the end of the list, the user will enter "-1" 
for the grade. 

Your program should validate that the grades are within a valid range of 0 to 100. 

The program should store the grades in a list. 

Calculate the following information based on the data:

- number of students who took the exam

- average grade on the exam 

- standard deviation of the grades 
  (see https://joannakl.github.io/core109_f17/slides/week6/grades.pdf if you do not
  remember how to calculate the standard deviation) 
  
Note: taking a square room of a number is the same as raising that number 
to exponent equal to 0.5. If you want to take a square root of x in Python, you can 
calculate:
    sqrt_x = x ** 0.5 
  


==============
Here are a few sample runs of your program:
    
Enter the grades one at a time. Enter -1 to indicate the end.

next grade: 90

next grade: -7
invalid score, not counted

next grade: 70

next grade: 85

next grade: -1

 ----------
list of grades:  [90, 70, 85]
number of grades:  3
average score:     81.66666666666667
standard dev:      8.498365855987975

===

Enter the grades one at a time. Enter -1 to indicate the end.

next grade: -1

 ----------
list of grades:  []
number of grades:  0

==============

__COMMENT YOUR SOURCE CODE__ by 

* briefly describing parts of your program 
* including your name at the top of your
  file (above these instructions)
"""