Letter Grade

A teacher marks tests out of 100 and reports a letter grade. Write letter_grade(score) that returns the grade as a string:

• 90 to 100: 'A'
• 80 to 89: 'B'
• 70 to 79: 'C'
• 60 to 69: 'D'
• 0 to 59: 'F'

A score below 0 or above 100 is a typing mistake: return 'Invalid' for it.

Examples

letter_grade(95)   # returns 'A'
letter_grade(83)   # returns 'B'
letter_grade(40)   # returns 'F'
letter_grade(120)  # returns 'Invalid'

Note

score is always a whole number.