def letter_grade(number_grade):
    if number_grade >= 90:
        return 'A'
    elif number_grade >= 80:
        return 'B'
    elif number_grade >= 70:
        return 'C'
    elif number_grade >= 60:
        return 'D'
    return 'F'


def main():
    #print("Hello, world")
    num_grade = 65
    let_grade = letter_grade(num_grade)
    print(let_grade)


if __name__ == "__main__":
    main()