package main

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

var sc = bufio.NewScanner(os.Stdin)

func nextInt() int {
        i, e := strconv.Atoi(nextString())
        if e != nil {
                panic(e)
        }
        return i
}

func nextInt64() int64 {
        i, e := strconv.ParseInt(nextString(), 10, 64)
        if e != nil {
                panic(e)
        }
        return i

}

func nextFloat() float64 {
        f, e := strconv.ParseFloat(nextString(), 64)
        if e != nil {
                panic(e)
        }
        return f
}

func nextString() string {
        sc.Split(bufio.ScanWords)
        sc.Scan()
        return sc.Text()
}

func main() {
        A, B, X, Y := nextFloat(), nextFloat(), nextFloat(), nextFloat()
        if X*B/A > Y {
                fmt.Println(Y + Y*A/B)
        } else {
                fmt.Println(X + X*B/A)
        }
}