#!/usr/bin/env python3
# DFS from unvisted vertices, marking everything below as non-root.
# Stop once reaching an already twice visited vertex.
# (Visiting the 2nd time is needed to find possible currently-marked-as-root
# vertices that need to be unmarked.)

__import__("sys").setrecursionlimit(int(1.1e5))

# Count the number of root SCCs
n, m = map(int, input().split())
g, root, vs, cnt = [[] for _ in range(n)], [True] * n, [0] * n, [0] * n
[g[a].append(b) for _ in range(m) for a, b in [[int(i) - 1 for i in input().split()]]]


def visit(u, r):
    for v in g[u]:
        if vs[v] == r or cnt[v] == 2: continue
        vs[v], root[v], cnt[v] = r, False, cnt[v] + 1
        visit(v, r)


for u in range(n):
    if vs[u] == 0:
        vs[u], cnt[u] = (r := u + 1), cnt[u] + 1
        visit(u, r)

print(sum(root))