import math._

object Main {
  def main(args: Array[String]) {
    val sc = new java.util.Scanner(System.in)
    val x1, y1, x2, y2 = sc.nextDouble

    val x = x1 + x2
    val y = y2 - y1
    val q = x1 * y / x
    println(y1 + q)
  }
}