import scala.io.Source
import scala.math.sqrt

object Main extends App {
  val Array(x1: Double, y1: Double, x2: Double, y2: Double) = Source.stdin.getLines.next.split(" ").map(_.toDouble)

  val v = sqrt((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1))
  println(v)
}