object Main {
  def bfs(r: Int, u: Int, w: Int, h: Int, a: Int, b: Int): BigInt = {
    if(r == w && u == h)
      1
    else if(r > w || u > h)
      0
    else if(r <= b && u >= h - a + 1)
      0
    else
      bfs(r, u+1, w, h, a, b) + bfs(r+1, u, w, h, a, b)
  }

  def main(args: Array[String]):Unit = {
    val n = io.StdIn.readLine.split(" ").map(_.toInt)
    val ba = (Math.pow(10, 9) + 7).toInt
    println(bfs(1, 1, n(0), n(1), n(2), n(3)))
  }
}