score=input("Enter Score : ")
try :
    s=float(score)
except :
    print("Bad score")
    quit()

if s>1.0 or s<0.0 :
    print("Bad score")
elif s>=0.9 :
    print("A")
elif s>=0.8 :
    print("B")
elif s>=0.7 :
    print("C")
elif s>=0.6 :
    print("D")
elif s<0.6 :
    print("F")
