COMPUTE f(n) giving wrong answer

[S18LPC-OL0069 (Piyush Gupta)]

http://ide.codingblocks.com/#/s/17428

The code is giving the right answer on my system. But the online judge shows wrong answer.

[Prateek-Narang-10209158320224419 (Prateek Narang)]

Take long long int everywhere.