AT-OEMO-REG 2001 Problem 1

Let $n$ be an integer. We consider $s (n)$ , the sum of the $2001$ powers of $n$ with the exponents $0$ to $2000$ . So $s (n) = \sum_{k=0}^{2000}n ^k$ . What is the unit digit of $s (n)$ in the decimal system?