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

int main(void)
{
	double x1,x2,y1,y2,z1,z2,distance;

	scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
	z1=pow((x1-x2),2);
	z2=pow((y1-y2),2);
	distance=sqrt(z1+z2);

	printf("%f\n",distance);

}

