/bump...

n=1 or 4

$n$ and $n+1$ has only got prime factors $2$ and $5$. Since there's one odd and one even among $n$ and $n+1$, the odd one must be $5^a$ and so the even one isn't divisible by $5$, so it is $2^b$. Now look here: #9 and #11 And here.