class Point
  def initialize(x, y)
    @x = x
    @y = y
  end

  attr_reader :x, :y

  def distance(p)
    (((@x - p.x) ** 2.0 + (@y - p.y) ** 2.0) ** 0.5)
  end
end

a,b,c,d=gets.split.map &:to_f
p1=Point.new(a,b)
p2=Point.new(c,d)
puts p1.distance(p2)

