Roots of Unity

Let $a_1,a_2,\dots,a_n$be the $n^\text{th}$roots of unity and let be the roots of unity. Compute $\frac{a_1a_2\cdots a_n}{b_1b_2\cdots b_{n+1}}.$ 

The hint is: What is the $n^\text{th}$root of unity?