Integer Sequence

Question

$\begin{align*} &\text{A} = \text{Set of integer sequence }\left ( a_1,a_2,a_2 ,\dots , a_k \right ) \\ &\text{where } 1 \leq a_1 \leq a_2 \leq a_3 \leq \dots \leq a_k \leq n \\ &\text{B} = \text{Set of integer sequence }\left ( b_1,b_2,b_2 ,\dots , b_k \right ) \\ &\text{where } 1 \leq b_1 < b_2 < b_3 < \dots < b_k \leq n \\ \end{align*}$

Prove that $\begin{align*} |A| = |B| \end{align*}$