"""Validate the input provided by the user using a while loop"""

def letter_grade(score) :

    if score < 60 :
        grade = 'F'
    elif score < 70 :
        grade = 'D'
    elif score < 80 :
        grade = 'C'
    elif score < 90 :
        grade = 'B'
    else :
        grade = 'A'

    return grade

if __name__ == '__main__' :

    score = input("enter score: ")
    while score < 0 or score > 100 :
        print 'ERROR: the score cannot be negative or greater than 100'
        score = input("enter score: ")
    grade = letter_grade(score)
    print "the letter grade is:", grade