#!/usr/bin/env ruby

require 'optimist'

conf = Optimist::options do
  banner "stddev [-r <d>] < <one number per line>"
  opt :round, "Number of digits after decimal point.", :type => :int, :default => -1
  opt :corrected, "corrected stddev", :type => :bool, :default => false
end

sum = 0.0
i = 0
cached = []
while line=STDIN.gets
  v = line.to_f
  sum += v
  cached << v
  i +=1
end

avg = sum/i.to_f

var = 0
cached.each { |v|
  var += (avg - v)**2
}

stddev = 0
if conf[:corrected]
  stddev = Math.sqrt(var/(i.to_f-1))
else
  stddev = Math.sqrt(var/i.to_f)
end

if conf[:round] >= 0
  puts stddev.round conf[:round]
else
  puts stddev
end