#include<stdio.h>
#include<math.h>

int main(void){
	double x0,x1,y0,y1;
	double dis;
	
	scanf("%lf %lf %lf %lf",&x0,&y0,&x1,&y1);
	dis=sqrt(pow(x1-x0,2)+pow(y1-y0,2));
	printf("%.8f\n",dis);
	
	return 0;
}