package main

import "fmt"

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

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