score = raw_input("Enter Score: ")
score=float(score)
if score <= 1:
	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")
else :
	print("Score must be a number between 0 and 1")