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

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