package main

import (
	"fmt"
	"strconv"
)

func main() {
	var px, py, qx, qy int
	fmt.Scan(&px, &py, &qx, &qy)

	x_dif := abs(px - qx)
	y_dif := abs(py - qy)

	t2 := x_dif + y_dif
	t := t2 / 2
	ans := strconv.Itoa(t)
	if t2%2 != 0 {
		ans += ".5"
	}
	fmt.Println(ans)
}

func abs(a int) int {
	if a < 0 {
		return a * -1
	}
	return a
}