In the following expression, a and n are integers:

(a-1)ⁿ(a-3)²ⁿ(a-5)³ⁿ(a-7)⁴ⁿ...(a-197)⁹⁹ⁿ(a-199)¹⁰⁰ⁿ

When does the expression have a negative value? You should carefully state conditions on both a and n.