LetterGrade
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At the end of every term, Prof ? has to compute the final
grades for the students in his CSE 1020 course.  One of
the steps involves converting a percentage score to a 
letter score, according to the following table

>= 90           A+
>= 80 and < 90  A
>= 75 and < 80  B+
>= 70 and < 75  B
>= 65 and < 70  C+
>= 60 and < 65  C
>= 55 and < 60  D+
>= 50 and < 55  D
>= 40 and < 50  E
< 40            F

Input
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The input consists of a single real number r, with 
0 <= r <= 100.

Output
------
The output consists of a string (A+, A, B+, B, C+, C, D+, 
D, E or F) that corresponds to the input.

Sample input
------------
77.77

Sample output
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B+