#!/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
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
}

var /= i.to_f

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