#include<stdio.h>
int abs(int n)
{
	if (n < 0)
		n *= -1;
	return n;
}
int main()
{
	int px, py, qx, qy;
	scanf("%d %d", &px, &py);
	scanf("%d %d", &qx, &qy);
	printf("%.20lf\n", (double)(abs(px - qx) + abs(py - qy)) / 2);
	return 0;
}