def convert(grade):
    if grade >= 80:
        return 'A'
    elif grade >= 70:
        return 'B'
    elif grade >= 60:
        return 'C'
    elif grade >= 50:
        return 'D'
    else:
        return 'F'


for name, results in read_grades("math.txt").items():
    print(name + " " + convert(mean(results)))