I read several articles on bit manipulation.But I still can't solve problems on bit manipulation.I am too weak at this.How can I improve myself in this topic or what should I learn and where can be my lacking? Any article or list of problems according to hardness level or suggestion will be helpful on this topic.

Whenever I try to build and run the code it shows "the system can't find the file specified".What's the problem?Please help!

I am gradually going down.Maybe my rating is the last in codeforces. But my max was specialist without any practice. And now I am working hard(possible) but my rating is going down.Please seniors give me some tips.Where is my lacking?