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

main(){
	double x1,y1,x2,y2;
	double x,y,d;
	
	scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
	x = x2 - x1;
	y = y2 - y1;
	d = sqrt(x * x + y * y);
	printf("%.5f\n",d);
	return 0;
}