package main

import "fmt"

func abs(a float64) float64 {
	if a >= 0 {
		return a
	}
	return -a
}

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