i need to put some advertisement to my blog
You are misinformed. Purchasing a domain name a domain mapping changes nothing. It does not allow for blogger initiated advertising.
Before Upgrading – Important Notes
Same Rules Apply – The Domain Mapping Upgrade does not enable the permission to use advertising, any kind of prohibited code, or upload additional themes or plugins. With the upgrade, your blog will still be hosted here at WordPress.com, which means that you will not have FTP access to your files and you will still be required to abide by our Terms of Service. http://en.support.wordpress.com/domain-mapping/
No blogger initiated advertising, retailing or reselling the work created or services provided by anyone other than yourself is allowed on free hosted wordpress.COM blogs. E-commerce transactions via shopping carts and the like cannot be conducted on free blogs from and being free hosted by wordpress.COM
The advertising exceptions noted in support documentation are for high traffic blogs that qualify for and are accepted into the Ad Control program, and for extremely high traffic blogs that qualify and are accepted into the paid VIP hosting program.
If you have any further questions about advertising and/or affiliate links and/or ecommerce on a free blog from and being free hosted by wordpress.COM please contact Staff directly using this link. http://en.support.wordpress.com/contact/
If you do require an ecommerce site, advertising and/or affiliate links you can hire a web host and get a free software install from http://wordpress.org Refer to > http://support.wordpress.com/com-vs-org/