object Main {
  def main(args: Array[String]): Unit = {
    val scanner = new java.util.Scanner(System.in)
    val x1, y1, x2, y2 = scanner.nextDouble
    val xd = x2 - x1
    val yd = y2 - y1
    println(f"${math.sqrt(xd * xd + yd * yd)}%.5f")
  }
}

