AT-OEMO-FED1 2006 Problem 1

Let $n$ be a non-negative integer, which ends written in decimal notation on exactly $k$ zeros, but which is bigger than $10^k$ . For a $n$ is only $k=k(n)\geq2$ known. In how many different ways (as a function of $k=k(n)\geq2$ ) can $n$ be written as difference of two squares of non-negative integers at least?