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'