package main

import . "fmt"
import . "math"
import "math/rand"

func main() {
	var xa,ya,xb,yb float64
	Scan(&xa,&ya,&xb,&yb)
	var mind float64 = 1e9
	var ans float64
	for i := 0; i < 1e7; i++ {
		yp := rand.Float64()*Abs(ya-yb)+Min(ya,yb)
		d := Hypot(xa,ya-yp)+Hypot(xb,yb-yp)
		if d < mind {
			ans,mind = yp,d
		} 
	}
	for i := 0; i < 1e7; i++ {
		yp := ans+2*rand.Float64()-1
		d := Hypot(xa,ya-yp)+Hypot(xb,yb-yp)
		if d < mind {
			ans,mind = yp,d
		} 
	}
	for i := 0; i < 1e7; i++ {
		yp := ans+rand.Float64()-0.5
		d := Hypot(xa,ya-yp)+Hypot(xb,yb-yp)
		if d < mind {
			ans,mind = yp,d
		} 
	}
	Println(ans)
}