sco=input("Enter your score:")

try:
    s=float(sco)
except:
    print("Bad score")
    quit()

def computegrade():
    if s>=0.9 and s<1.0:
        return "A"
    elif s>=0.8 and s<0.9:
        return "B"
    elif s>=0.7 and s<0.8:
        return "C"
    elif s>=0.6 and s<0.7:
        return "D"
    elif s<0.6 and s>0:
        return "F"
    else: print("bad score")
    quit()

print(computegrade())