#include<stdio.h>
#include<math.h>
int main(void)
{
	double xa,xb,ya,yb,len;
	scanf("%lf %lf %lf %lf",&xa,&ya,&xb,&yb);
	len=sqrt((xb-xa)*(xb-xa)+(yb-ya)*(yb-ya));
	printf("%lf\n",len);
	return 0;   
}