write a program that prompts for and accepts input of test ggrades that are integers between 0 and 100. For each numerical test grade, your program should display a corresponding letter grade based on the following rules
if the grade is between 90 and 100 inclusive, display an 'A'.
if the grade is between 80 and 89 inclusive, display a 'B'.
if the grade is between 65 and 79 inclusive, display a 'C'.
if the grade is between 50 and 64 inclusive, display a 'D'.
if the grade is less than 50, display an 'F'.
The program should continue to read and classify the test grades until the user enters -1 as a sentinel value.