Suppose  R(k) = \{(i,j): a_{i,j} \leq k \} . Why does  |R(k)|= \left \lfloor \frac{k}{1} \right \rfloor  + \left \lfloor \frac{k}{2} \right \rfloor+ \cdots + \left \lfloor \frac{k}{k} \right \rfloor , where  k is a positive integer?