object Main extends App {
  val sc = new java.util.Scanner(System.in)
  val p1 = List(sc.nextDouble(), sc.nextDouble())
  val p2 = List(sc.nextDouble(), sc.nextDouble())

  import scala.math.{pow, sqrt}

  val d = sqrt(pow(p1(0) - p2(0), 2.0) + pow(p1(1) - p2(1), 2.0))
  println(f"$d%.8f")
}