package main

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

var bufin *bufio.Reader
var bufout *bufio.Writer

func main() {
	bufin = bufio.NewReader(os.Stdin)
	bufout = bufio.NewWriter(os.Stdout)
	defer bufout.Flush()
	var vl, vr, d, w float64
	fmt.Fscanf(bufin, "%f %f\n", &vl, &vr)
	fmt.Fscanf(bufin, "%f\n", &d)
	fmt.Fscanf(bufin, "%f\n", &w)
	var t = d / (vl + vr)
	fmt.Fprintf(bufout, "%.10f\n", t*w)
}