#include<stdio.h>
#include<math.h>
int main(){
	
	double x1,x2,y1,y2;
	double kyori;
	
	scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
	
	kyori=sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
	
	printf("%.8f\n",kyori);
	return 0;
}