#include <stdio.h>
#include <math.h>

int main(void) {
	int px, py;
	int qx, qy;
	int dx, dy;
	scanf("%d%d", &px, &py);
	scanf("%d%d", &qx, &qy);
	dx = abs(px - qx);
	dy = abs(py - qy);
//	printf("dx=%d\ndy=%d\n", dx, dy);
	if((dx + dy) % 2 == 1){
		printf("%.1f\n", dx / 2.0 + dy / 2.0);
	}
	else{
		printf("%d\n", (int)(dx / 2.0 + dy / 2.0));
	}
	return 0;
}