#include<bits/stdc++.h>
using namespace std;
double ax,ay,bx,by;
int main(){
	cin>>ax>>ay>>bx>>by;
	double dx=bx-ax,dy=by-ay;
	double res=sqrt(dx*dx+dy*dy);
	cout<<fixed<<setprecision(3)<<res;
	return 0;
}