COMPUTE F(N) C++ online assignment 1

time limit exceeded eroor how can i correct that ?
https://ide.codingblocks.com/#/s/22986

Please share link to question also from next time.

In this question you need not actually calculate each term.
You can easily find a relation of the terms. The odd terms and the even terms follow
different particular relations. Try to figure that out. Then you can solve it very easily.