#include<stdio.h>
#include<math.h>
int main() {
	double x1, x2, y1, y2;
	double ans;

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

	ans = (double)sqrt(pow(x1 - y1, 2.0) + pow(x2 - y2, 2.0));

	printf("%.15f", ans);

	return 0;
}