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