#!/usr/bin/ruby
# -*- coding: utf-8 -*-
=begin

Discussion Solution Statistics Submit

Math Functions - Distance
Time Limit : 1 sec, Memory Limit : 65536 KB
?????¢

?????? P1(x1, y1), P2(x2, y2) ????????¢????±???????????????°????????????????????????
Input

x1, y1, x2, y2 ????????°???????????????????????§?????????????????????
Output

P1??¨P2????????¢????????°??§?????????????????????????????????0.0001??\????????????????????£????????????????????¨????????????
Sample Input

0 0 1 1

Sample Output

1.41421356

=end
fh=STDIN
fl=fh.gets.chomp.split
x1=fl[0].to_f
y1=fl[1].to_f
x2=fl[2].to_f
y2=fl[3].to_f
r = Math.sqrt((x1-x2)**2 + (y1-y2)**2)
puts r