#include<bits/stdc++.h>
	using namespace std;
	int main()
	{
	double a1,a2,b1,b2;
	cin>>a1>>a2>>b1>>b2;
	printf("%.3f",sqrt((a1-a2)*(a1-a2)+(b1-b2)*(b1-b2)));
	return 0;
	}