There is the following code:

```
# API controller for authentication
class Api::V1::SessionsController < Api::V1::ApplicationController
  skip_before_action :authorize

  def create
    @user = User.find_by(email: params[:user][:email])
    unless @user && @user.authenticate(params[:user][:password])
      @error_message = 'Invalid username or password'
      render 'shared/error', status: :unauthorized
    end
  end
end

```

I use Rubocop to check my code if it matches to Ruby guidelines. I got the following error:

```
Use a guard clause instead of wrapping the code inside a conditional expression.
    unless @user && @user.authenticate(params[:user][:password])

```

So, I don't understand how I can make this code better using guard clause. Thanks in advance!