Burles

There is an $n×m$ grid. You are standing at cell $(1,1)$ and your goal is to finish at cell $(n,m)$. You can move to the neighboring cells to the right or down. In other words, suppose you are standing at cell $(x,y)$. You can: move right to the cell $(x,y+1)$ — it costs $x$ burles; move down to the cell $(x+1,y)$ — it costs $y$ burles. Can you reach cell $(n,m)$ spending exactly $k$ burles? ##