import java.util.Scanner

object Problem236 {
  def main(args: Array[String]) {
    val sc = new Scanner(System.in)
    val (a, b, x, y) = (sc.nextDouble, sc.nextDouble, sc.nextDouble, sc.nextDouble)

    val result = x * b / a < y match {
      case true => x + x * b / a
      case false => y + y * a / b
    }

    println(result)
  }
}