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

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

	ans = sqrt((pow(x2 - x1, 2)) + (pow(y2 - y1, 2)));

	printf("%f", ans);

	return 0;
}