package main

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

// 参考： http://qiita.com/tnoda_/items/b503a72eac82862d30c6

// Max 64k
var sc = bufio.NewScanner(os.Stdin)

func nextLine() string {
	// sc.Split(bufio.ScanLines) // split \n
	sc.Scan()
	return sc.Text()
}
func nextInt() int {
	// sc.Split(bufio.ScanWords) // split SP
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}
func nextFloat() float64 {
	// sc.Split(bufio.ScanWords) // split SP
	sc.Scan()
	i, e := strconv.ParseFloat(sc.Text(), 64)
	if e != nil {
		panic(e)
	}
	return i
}

// Over 64k
var rdr = bufio.NewReaderSize(os.Stdin, 1000000)

func readLine() []byte {
	buf := make([]byte, 0, 1000000)
	for {
		l, p, e := rdr.ReadLine()
		if e != nil {
			panic(e)
		}
		buf = append(buf, l...)
		if !p {
			break
		}
	}
	return buf
}

func main() {
	sc.Split(bufio.ScanWords)
	v1, v2, d, w := nextFloat(), nextFloat(), nextFloat(), nextFloat()
	t := d / (v1 + v2)
	fmt.Println(w * t)
}