// No.410 出会い
package main

import (
	"fmt"
	"math"
)

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

	dx := math.Abs(float64(px)-float64(qx)) / 2
	dy := math.Abs(float64(py)-float64(qy)) / 2
	fmt.Println(dx + dy)
}