[Rails] Installing rails on Ubuntu

Hello, I'm trying to install the rails on ubuntu 11.10, but when I put
the command

felipe@Felipe:~/Downloads/rubygems-1.8.15$ sudo gem install rails
--no-r1 --no-rdoc
ERROR: Loading command: install (LoadError)
no such file to load -- zlib
ERROR: While executing gem ... (NameError)
uninitialized constant Gem::Commands::InstallCommand
felipe@Felipe:~/Downloads/rubygems-1.8.15$

it return this error, how can I install correctly.

Thanks

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