MODE = "stdio"
require 'matrix'
line = Array.new

if(defined? MODE)
  line = STDIN.read.split("\n")
else
  tmp = '0 1.5 1 1'.split("\n")
  tmp.each{|s|
    line << s.strip
  }
end


x1,y1,x2,y2 = line[0].split(" ").map(&:to_f)

d =  ((x2-x1)**2 + (y2-y1)**2)**(0.5)

printf("%.10f\n",d)