package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

func main() {
	var sc = bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Scan(); x1, _ := strconv.ParseFloat(sc.Text(), 64)
	sc.Scan(); y1, _ := strconv.ParseFloat(sc.Text(), 64)
	sc.Scan(); x2, _ := strconv.ParseFloat(sc.Text(), 64)
	sc.Scan(); y2, _ := strconv.ParseFloat(sc.Text(), 64)

	fmt.Println((y2 - y1) * x1 / (x1 + x2) + y1)
}