[9498] 시험 성적

score = int(input())

if score >= 90 and score <=100:
    print('A')
elif score >= 80 and score <=89:
    print('B')
elif score >= 70 and score <=79:
    print('C')
elif score >= 60 and score <=69:
    print('D')
else :
    print('F')