#include<bits/stdc++.h>
using namespace std;
int main()
{
	double Xa,Ya,Xb,Yb;
	scanf("%lf%lf%lf%lf",&Xa,&Ya,&Xb,&Yb);
	printf("%.3f",sqrt((Xb-Xa)*(Xb-Xa)+(Yb-Ya)*(Yb-Ya)));
	return 0;
}