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

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

	scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);

	distance = sqrt((x2 - x1)*(x2 - x1) + (y2 - y1)*(y2 - y1));

	printf("%f", distance);
	
	return 0;
}
