Ruby on Rails

Hi, guys,
I got the below error message when i  typed this command in terminal "rackup config.ru" ,  the file config.ru is rails 's configuration file.

You have already activated rack 1.3.2, but your Gemfile requires rack 1.2.3. Consider using bundle exec. (Gem::LoadError)

So,  how should i do , thanks a lot.

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