object Main {
  def main(args: Array[String]): Unit = {
    val Array(x1, y1, x2, y2, _*) = io.StdIn.readLine().split(" ").map(_.toDouble)

    val dx = Math.abs(x2 - x1)
    val dy = Math.abs(y2 - y1)
    val d = Math.sqrt(dx * dx + dy * dy)
    println(f"$d%.8f")
  }
}