inp = 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')