#include<bits/stdc++.h>
using namespace std;
double xa,xb,ya,yb;
int main()
{
	cin>>xa>>ya>>xb>>yb;
	double aa=pow((xa-xb),2);
	double bb=pow((ya-yb),2);
	double ans=sqrt(aa+bb);
	printf("%.3f",ans);
}