changed /etc permission in oneiric (ubuntu11.10) by using "sudo chmod 665 /etc" cant use sudo now

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http://askubuntu.com – i screwed up big time. i changed the permission of /etc in ubuntu 11.10 by using following command sudo chmod 665 /etc now when ever i try to use sudo it gives the following error: sudo: can't open /etc/sudoers: Permission denied sudo: no valid sudoers sources found, quitting any help will be greatly appreciated because i am new to ubuntu....i have already tried using su but it doesnt even login as root and even i give it the right password it always say Authentication Failure......i dont want to login as root coz its risky and even a newbie in linux like me is not stupid enough to have su (HowTos)