Replacing a featured image by an ads

Hello everyone,

I want find a solution to add an ads in place of a featured image, this is the structure of the home page of my site:

12
34
56
78
910

1 to 10 are recently added posts, i want to replace the featured image of the second post by an ads, so only 9 posts will be shown + 1 ads.

I will be glad is someone help me to sort this out, my coding skills are very limited.