#include <cstdio>

int main() {
	int ax, ay, bx, by;
	scanf("%d %d", &ax, &ay);
	scanf("%d %d", &bx, &by);

	ax = bx - ax;
	ay = by - ay;

	if (ax < 0) ax *= -1;
	if (ay < 0) ay *= -1;

	double res = (ax + ay) / 2.0;

	printf("%.20lf\n", res);

	return 0;
}