#!/usr/bin/env python
# -*- coding: utf-8 -*-
p = list(map(int,input().split()))
q = list(map(int,input().split()))
# p=[1,1]
# q=[0,0]
r = ((q[0]-p[0]) ** 2 + (q[1] - p[1]) ** 2)**(1/2)
print(r/2)