user_score = float(input('Enter your grade:'))

def return_grade(user_input):
    try:
        if user_score > 1.0 or user_score < 0.0 :
            return ('Please enter a number between 0.0 and 1.0')

        elif user_score >= 0.9 :
            return 'A'
        elif user_score >= 0.8 :
            return 'B'
        elif user_score >= 0.7 :
            return 'C'
        elif user_score >= 0.6 :
            return 'D'
        elif user_score < 0.6 :
            return 'F'
    except VelueError:
        print('Please enter a number')

print(return_grade(user_input))