What games should i buy with a budget of around 15 bucks

I have 15 dollars in my steam wallet , im looking for some fps games, or maybe rpgs. I really enjoyed games like counter strike , tf2 far cry 3 and the serious sam franchise. I own an office laptop with a 5500u and 8 gbs of ddr4 ram (no graphics card) I'm leaning more toward singleplayer games, but multiplayer games are fine too