\begin{gather*}
\text{SAD} = \sum_{\text{block}}|S_k - S_{k-1}| = \sum_{\text{row_1}}|S_k - S_{k-1}| + \sum_{\text{row_2}}|S_k - S_{k-1}| \\
\sum_{\text{block}}|S_k - S_{k-1}\| \geq \|{\sum_{\text{row_1}}S_k - \sum_{\text{row_1}}S_{k-1}}\| + |{\sum_{\text{row_2}}S_k - \sum_{\text{row_2}}S_{k-1}}| \\
\sum_{\text{block}}|S_k - S_{k-1}| \geq \sum\limits_{i=1}^2|{\sum{\text{row_i}}S_k - \sum_{\text{row_i}}S_{k-1}}|\\
\end{gather*}