

object Main extends App{
  
  override def main(args :Array[String]):Unit = {
    
    val input = io.StdIn.readLine.split(' ')
    
    val a:point = point(input(0).toDouble, input(1).toDouble)
    val b:point = point(input(2).toDouble,input(3).toDouble)
    
    val length = a.distanceFrom(b)
    
    println(length)
  }
  
  
  case class point(x: Double, y: Double){
    def distanceFrom(p: point): Double = {
      Math.sqrt((x - p.x) * (x-p.x) + (y-p.y)*(y-p.y)) 
    }
  }
  
}