For some reason I keep getting an error message when trying to print the std dev...... Can anyone shed any light on this?

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