Write a program that reads an exam score and prints the matching grade.
A score of 90 to 100 is A, 80 to 89 is B, 70 to 79 is C, 60 to 69 is D, and any lower score is F.
The first line contains the exam score. The score is an integer greater than or equal to 0 and less than or equal to 100.
Print the exam grade on the first line, as one of A, B, C, D, F.