inp = raw_input('Enter score: ')
try:
    score = float(inp)
except:
    score = -1

if score > 1.0 or score < 0.0:
    print 'Bad score'
elif score > 0.9:
    print 'A'
elif score > 0.8:
    print 'B'
elif score > 0.7:
    print 'C'
elif score > 0.6:
    print 'D'
else:
    print 'F'