키 순서

• N의 범위가 다른 것을 제외하고는 Cow Contest과 같은 문제이다. 풀이는 그쪽 참고.

"""Solution code for "BOJ 2458. 키 순서".

- Problem link: https://www.acmicpc.net/problem/2458
- Solution link: http://www.teferi.net/ps/problems/boj/2458

Tags: [Floyd]
"""

import sys


def main():
    N, M = [int(x) for x in sys.stdin.readline().split()]
    higher_rankers = [set() for _ in range(N)]
    lower_rankers = [set() for _ in range(N)]
    for _ in range(M):
        a, b = [int(x) for x in sys.stdin.readline().split()]
        higher_rankers[a - 1].add(b - 1)
        lower_rankers[b - 1].add(a - 1)

    for i in range(N):
        for higher_ranker in higher_rankers[i]:
            lower_rankers[higher_ranker].update(lower_rankers[i])
        for lower_ranker in lower_rankers[i]:
            higher_rankers[lower_ranker].update(higher_rankers[i])

    answer = sum(1 for i in range(N)
                 if len(higher_rankers[i]) + len(lower_rankers[i]) == N - 1)
    print(answer)


if __name__ == '__main__':
    main()