package main

import . "fmt"
import . "math"

func main() {
	var xa,ya,xb,yb float64
	Scan(&xa,&ya,&xb,&yb)
	if ya > yb {
		xa,ya,xb,yb = xb,yb,xa,ya
	}
	p := 1.0
	for yb < 1e7 {
		p *= 10
		ya *= 10
		yb *= 10
	}
	var mind float64 = 1e9
	var ans float64
	for yp := int64(ya); yp <= int64(yb); yp++ {
		d := Hypot(xa*p,(ya-float64(yp)))+Hypot(xb*p,(yb-float64(yp)))
		if d < mind {
			mind,ans = d, float64(yp)/p
		}
	}
	Println(ans)
}