P2437 Bee Routes

A bee crawls on a numbered honeycomb as shown in the figure below. It can only move from a cell with a smaller label to an adjacent cell with a larger label. How many different routes are there for the bee to crawl from cell $m$ to cell $n$, with $m < n$? (Note: The original statement contains an error; the top-right corner should be labeled $n-1$.) 

A single line containing integers $m$ and $n$.

Output the number of possible crawling routes.

Constraints: For $100\%$ of the testdata, $1 \le m, n \le 1000$. Translated by ChatGPT 5