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