class Expr
  def initialize(left, op, right)
    @left = left
    @op = op
    @right = right
  end
end
class Expr
  def to_s
    s = "("
    s += @left.to_s
    s += @op
    s += @right.to_s
    s + ")"
  end
end
class Expr
  def to_f
    left = @left.to_f
    right = @right.to_f
    if @op == "+" then
      left + right
    elsif @op == "-" then
      left - right
    elsif @op == "*" then
      left * right
    elsif @op == "/" then
      left / right
    else
      0
    end
  end
end