Write a program that computes the average test score over an unspecified number of tests as follows: $(1)$ Ask the user to enter numeric

grades; $(2)$ as long as there are more grades, add them to the total; $(3)$ use a counter variable to track the number of grades entered by the

user; $(4)$ stop when the user enters a negative score; $(5)$ output the average grade for the class.

Sample run of program:

Enter a numeric grade:

$>78$

Enter a numeric grade:

$>89$

Enter a numeric grade:

$>77$

Enter a numeric grade:

$>90$

Enter a numeric grade:

$>97$

Enter a numeric grade:

$>92$

Enter a numeric grade:

$>-1$

The average grade is $87\mathrm{.}1667$