grades = [0, 6, 12, 18]
def grades_average(grades):
    sum_of_grades = grades_sum(grades)
    average = sum_of_grades / len(grades)
    return average
def grades_variance(scores, average):
    variance = 0
    for i in scores:
        variance += int((average - i) ** 2) 
    return variance / len(scores)
    
print grades_variance(grades, grades_average(grades))