====== 세진 바이러스 ======

===== 풀이 =====
  * [[ps:Strongly Connected Component]]를 활용하는 기본 문제. Condensation graph를 만들고 소스 노드들에 바이러스를 뿌리면, 모든 노드들로 전달시킬수 있다. 
  * 시간복잡도는 O(V+E).

===== 코드 =====
<dkpr py>
  """Solution code for "BOJ 15783. 세진 바이러스".

- Problem link: https://www.acmicpc.net/problem/15783
- Solution link: http://www.teferi.net/ps/problems/boj/15783

Tags: [SCC]
"""

import itertools
import sys
from teflib import graph as tgraph


def main():
    N, M = [int(x) for x in sys.stdin.readline().split()]
    graph = [[] for _ in range(N)]
    for _ in range(M):
        A, B = [int(x) for x in sys.stdin.readline().split()]
        graph[A].append(B)

    scc_graph, *_ = tgraph.condensation_graph(graph)
    answer = len(scc_graph) - len(set(itertools.chain.from_iterable(scc_graph)))
    print(answer)


if __name__ == '__main__':
    main()

</dkpr>
  * Dependency: [[:ps:teflib:graph#condensation_graph|teflib.graph.condensation_graph]]

{{tag>BOJ ps:problems:boj:플래티넘_4}}