inp = raw_input('Please give score between 0.0 and 1.0: ')
try:
    finp = float(inp)
except:
    finp = -1
if finp >= 0.0 and finp <= 1.0 :
    score = finp
else:
    print 'Error, please insert valid score'
    quit()

if score >= 0.9 :
    print 'A'
elif score >= 0.8 :
    print 'B'
elif score >= 0.7 :
    print 'C'
elif score >= 0.6 :
    print 'D'
elif score < 0.6 :
    print 'F'