linear algebra - Matrix's determinant

I've got to calculate determinant for such matrix:
$$ \left[

\begin{array}{cccc}
a_1+b & a_2 & \cdots & a_n\\
a_1 & a_2+b & \cdots & a_n\\
\vdots & \vdots & \ddots & \vdots\\
a_1 & a_2 & \cdots & a_n+b\\
\end{array}
\right]
$$
Please give me some tips how to calculate this.

Thanks in advance