一个求平均成绩的python的程序

total=0
counter=0

   grade=raw_input("enter grade:")
grade=int(grade)

while grade != -1:
  total=total+grade
  counter=counter+1
  grade=raw_input("enter grade:")
  grade=int(grade)

if counter != 0:
  average=float(total)/counter
  print average
else:
  print "No grade were entered"