/**
  * http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ITP1_10_A&lang=jp
  * Math Functions - Distance
  */
object Main {
  def main(args: Array[String]): Unit = {
    val Array(x1, y1, x2, y2) = io.StdIn.readLine().split(" ").map(_.toDouble)
    val d = math.sqrt(math.pow(x1 - x2, 2) + math.pow(y1 - y2, 2))
    println(d)
  }
}