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

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

	/* ??\??? */
	scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);

	/* 2??????????????¢?¨???? */
	distance = sqrt(pow((x2 - x1), 2) + pow((y2 - y1), 2));

	printf("%.7f\n", distance);

	return 0;
}