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

int main(){

	double x1, x2, y1, y2, a;

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

	a = (x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2);

	printf("%.11lf", sqrt(a));





	return 0;
}