1950 - [Ceoi2006]Link

给 N个点N 条边的有向图。 要求添最小的边，使得点1 到达其他所有点的最短路长度不超过K。

The first line of input contains two integers N and K (2 ≤ N ≤ 500 000, 1 ≤ K ≤ 20 000) – the number of pages and the target maximum number of links to be followed. Each of the following N lines contains two different integers A and B (1 ≤ A, B ≤ N) meaning that the link on page A points to page B.

The first and only line of output should contain a single integer, the minimum number of additional links required to make the website K-reachable.

one possible solution is to add the links 1→7, 1→14 and 14→10