Need help! My programm return wrong answer, butI can't understand why

```
grades = [100, 100, 90, 40, 80, 100, 85, 70, 90, 65, 90, 85, 50.5]
def grades_sum(grades):
total = 0
for grade in grades:
total += grade
return total
def grades_average(grades):
sum_of_grades = grades_sum(grades)
average = sum_of_grades / float(len(grades))
return average
def grades_variance(scores):
average = grades_average(grades)
variance = 0
for score in scores:
variance += (average - score) ** 2
res = variance / float(len(scores))
return res
print grades_variance(grades)
```