PL-OM 2016 Problem 3

Let $a, \ b \in \mathbb{Z_{+}}$ . Denote $f(a, b)$ the number sequences $s_1, \ s_2, \ ..., \ s_a$ , $s_i \in \mathbb{Z}$ such that $|s_1|+|s_2|+...+|s_a| \le b$ . Show that $f(a, b)=f(b, a)$ .