package main

import (
	"fmt"
)

func main() {
	var Px, Py, Qx, Qy float64
	fmt.Scan(&Px, &Py, &Qx, &Qy)
	fmt.Println((abs(Px-Qx)+abs(Py-Qy))/2)
}

func abs(x float64) float64 {
	if x < 0 {
		return x * -1
	}
	return x
}