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')

a4935dd2d9474d2b49ae28337302443a08afa83d
<mask token>

<mask token>
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')

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')
