import scala.io.StdIn
import Math.sqrt
/**
  * AOJ
  */
object Main {

  def main(args: Array[String]): Unit = {

    val i = StdIn.readLine().split(" ").map(_.toFloat)
    val x = i(0) - i(2)
    val y = i(1) - i(3)
    println(sqrt(x * x + y * y))
  }
}