Problem of insight debugger

Hello everybody,
I have a problem with installing the insight on my ubuntu 17.10.
So, first of all I added these two repostries to the /etc/apt/sources.list

 deb natty main 
deb-src natty main 

after that, I updated the system using apt-get update
and then I tried to install it apt-get install insight

I get this error while the update

W: GPG error: natty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 35DA01C261E46227
E: The repository ' natty Release' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

and this one trying to install the insight debugger

E: Package 'insight' has no installation candidate

I look for a solution almost anywhere in the internet but I didn’t find anything. I hope you can help me, thank you.

I’ve moved this to the category because this site isn’t designed to take tech support questions. Sorry. I’d recommend using one of the official support channels listed on the Finding Help page.