Maximum number of Bishops that can be placed on a NxN Chessboard - SPOJ

Using the Constant time formula for Maximum bishops on a chessboard, which is:

int maxBishops(N) return 2*(N-1);

Implemented for the value of N lesser than 10^100, as follows: http://ift.tt/1N4wdOc

Verified using Wolfram|Alpha but getting wrong answer on SPOJ's BISHOPS submission.

Am I missing something in the algorithm or is it an implementation issue?