// yukicoder: No.410 出会い
// 2019.4.15 bal4u

#include <stdio.h>

int main()
{
	int Px, Py, Qx, Qy;

	scanf("%d%d%d%d", &Px, &Py, &Qx, &Qy);
	Px -= Qx; if (Px < 0) Px = -Px;
	Py -= Qy; if (Py < 0) Py = -Py;
	if ((Px + Py) & 1) printf("%d.5\n", (Px + Py)/2);
	else printf("%d\n", (Px + Py) / 2);
	return 0;
}