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

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

	ans = (double)sqrt(x1 * x2 + y1* y2);

	printf("%lf", ans);

	return 0;
}