Hello , I would like some help regarding the problem UVA 616.This is a problem from math implementation section. I am at the moment checking for all possible values of group size ( till N) but its giving me TLE

The link :- https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=8&page=show_problem&problem=557