import math


def get_sd(mean, grade_list):
    """Calculate the standard deviation of the grades"""
    stan_dev = 0 # standard deviation
    sum_of_sqrs = 0
    for grade in grade_list:
        sum_of_sqrs += (grade - mean) ** 2
    stan_dev = math.sqrt(sum_of_sqrs / len(grade_list))
    return stan_dev