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

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