package main
import (
	"fmt"
	"math"
)

func main()  {
	ax, ay, bx, by := 0.0, 0.0, 0.0, 0.0
	fmt.Scan(&ax, &ay, &bx, &by)

	left := min(ay, by)
	right := max(ay, by)

	for i := 0 ; i < 500 ; i++ {
		m1 := (left * 2.0 + right) / 3.0
		m2 := (left + right * 2.0) / 3.0

		c1 := compute(m1, ax, ay, bx, by)
		c2 := compute(m2, ax, ay, bx, by)
		if c1 < c2 {
			right = m2
		} else {
			left = m1
		}
	}
	fmt.Printf("%.9f\n", (left+right)/2)

}

func compute(my, ax, ay, bx, by float64) float64 {
	return math.Sqrt(ax*ax+(ay-my)*(ay-my)) + math.Sqrt(bx*bx+(by-my)*(by-my))
}

func max(a, b float64) float64 {
	if a < b  {
		return b
	} else {
		return a
	}
}

func min(a, b float64) float64 {
	if a > b  {
		return b
	} else {
		return a
	}
}