#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int main(){
	double xa,xb,ya,yb;
	cin>>xa>>ya>>xb>>yb;
	double zjb1=xa-xb;
	double zjb2=ya-yb;
	double ab=sqrt(zjb1*zjb1+zjb2*zjb2);
	cout<<fixed<<setprecision(3)<<ab;
	return 0;
}