import scala.io.StdIn.readLine
import scala.math.{sqrt, pow}

case class Point(x: Double, y: Double) {
  def distance(other: Point): Double =
    sqrt(pow(x - other.x, 2) + pow(y - other.y, 2))
}

object Main extends App {
  val Array(x1, y1, x2, y2) = readLine.split(" ").map { _.toDouble }

  val p1 = Point(x1, y1)
  val p2 = Point(x2, y2)

  println(f"${p1 distance p2}%.8f")
}

