package main

import . "fmt"
import . "math/big"

func main() {
	var a,b int64
	x,y := new(Rat),new(Rat)
	Scan(&a,&b,x,y)
	ex := new(Rat).Mul(y, NewRat(a,b))
	ey := new(Rat).Mul(x, NewRat(b,a))
	if ex.Cmp(x) <= 0 {
		Println(ex.Add(ex,y).FloatString(20))
	} else {
		Println(ey.Add(ey,x).FloatString(20))
	}
}