package main

import "fmt"

func main() {
	var px, py int
	var qx, qy int
	fmt.Scan(&px, &py)
	fmt.Scan(&qx, &qy)
	fmt.Println(float64(abs(px - qx) + abs(py - qy)) / 2)
}

func abs(a int) int {
	if a < 0 {
		return -a
	}
	return a
}