Write a program the does the following:
1) Reads scores of three different classes
2) Checks if each score is legal within [0, 100]
3) If any score is illegal, quit program using "System.exit(0);"
4) Computes the average of all three scores
5) Assigns a letter grade {A, B, C, D, F} for the average score
6) Prints input scores, average and the letter grade.