#include 
#include <iostream>
#include <math.h>
using namespace std;
int main(){
	double x1,x2,y1,y2;
	cin>>x1>>y1>>x2>>y2;
	double a,b;
	a=x1-x2;
	b=y1-y2;
	double c;
	c=a*a+b*b;
	c=sqrt(c);
	printf("%.3lf\n",c);
}