# Write a function to calculate the standard deviation.
import math
import sys


def sd_calc(data):
    n = len(data)
    if n <= 1:
        return 0.0
    mean, sd = avg_calc(data), 0.0
    for el in data:
        sd += (float(el) - mean) ** 2
    sd = math.sqrt(sd / float(n - 1))
    return sd


def avg_calc(ls):
    n, mean = len(ls), 0.0
    if n <= 1:
        return ls[0]
    for el in ls:
        mean = mean + float(el)
    mean = mean / float(n)
    return mean
