What is the deal with this one?

I'm not able to figure out how to fix this.

334.071005917

Traceback (most recent call last):

File "python", line 32, in

File "python", line 30, in grades_std_deviation

TypeError: unsupported operand type(s) for ** or pow(): 'list' and 'int'

```
grades = [100, 100, 90, 40, 80, 100, 85, 70, 90, 65, 90, 85, 50.5]
def print_grades(grades):
for grade in grades:
print grade
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(scores)
variance = 0
for score in scores:
individual_variance = (average-score)**2
variance = variance + individual_variance
variance = variance / len(scores)
return variance
print grades_variance(grades)
def grades_std_deviation(variance):
return variance**0.5
print grades_std_deviation(grades)
```