P2002 Message Diffusion

This is the first problem of the contest. Here's an easy one—take these 100 points first.

There are $n$ cities connected by one-way roads. Messages spread along the roads. Given $n$ cities and the roads between them, ask for the minimum number of cities where you need to initially publish the message so that all $n$ cities receive it.

The first line contains two integers $n, m$, meaning $n$ cities and $m$ directed roads. In the next $m$ lines, each line contains two integers $b, e$, indicating there is a road from $b$ to $e$. Multiple edges and self-loops are allowed.

Output a single integer, the minimum number of cities where the message must be published.

- Sample Explanation #1 In the sample, publish the message in cities $4$ and $5$. - Constraints For $20\%$ of the testdata, $n \le 200$. For $40\%$ of the testdata, $n \le 2000$. For $100\%$ of the testdata, $1 \le n \le {10}^5$, $1 \le m \le 5 \times {10}^5$. Translated by ChatGPT 5