#!/usr/bin/env ruby

require 'trollop'


def usage
  STDERR.write "./stddev [-r <d>] < <one number per line>\n"
  exit 1
end
usage if not [0,2].include? ARGV.size

opts = Trollop::options do
  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.strip.to_f
  sum += v
  cached << v
  i +=1 
end

avg = sum/i.to_f

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

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