grade = int(input("Enter a student grade: "))

# see if grade >= 90
if grade >= 90:
    print("A")
# if it's not >= 90, do this
else:
	# check to see if it is >= 80
    if grade >= 80:               
        print("B")                
    # if it's not >= 80, do this
    else:
    	# check to see if it's >= 70
        if grade >= 70:
            print("C")      
        # if not, do this
        else:
        	# check to see if it's >= 65
            if grade >= 65:
                print("D")
            # if not, do this
            else: 
                print("F")