$p(n) = \sum _{ k=1 }^{ y }{ \left\lfloor \frac { n }{ { 5 }^{ k } } \right\rfloor }$
Given $$n,y,k$$ are positive integers, $$y$$ is the largest integer such that the inequality $$5^y \leq n$$ is satisfied. For the function $$p(n)$$ as described above, if $$p(n) = 781$$. Find the sum of all possible values of $$n$$.