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

int main()
{
	double x1,x2,y1,y2;
	double distance;
	
	scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
	printf("%lf\n",sqrt(pow(x1-x2,2)+pow(y1-y2,2)));

       return 0;
	
}