#!/usr/bin/env python3
import sys

sys.setrecursionlimit(int(2.1e5))

# Count the number of leaf SCCs using Tarjans algorithm.
N, m = map(int, input().split())
g = [[] for _ in range(N)]
for _ in range(m):
    a, b = map(lambda x: int(x) - 1, input().split())
    g[b].append(a)

counter = 0
n = [-1] * N
l = [-1] * N
vs = [False] * N
st = []
ans = 0

com = [-1] * N
C = 0


def visit(u):
    global counter, ans, C, st, n, l
    l[u] = n[u] = counter
    st.append(u)
    counter += 1
    vs[u] = True
    for v in g[u]:
        if n[v] == -1:
            visit(v)
        if vs[v]:
            l[u] = min(l[u], l[v])
    if l[u] == n[u]:
        cc = []
        while True:
            v = st[-1]
            st.pop()
            vs[v] = False
            com[v] = C
            cc.append(v)
            if u == v:
                break

        leaf = True
        for v in cc:
            for w in g[v]:
                if com[w] != C:
                    leaf = False

        if leaf:
            ans += 1

        C += 1


for u in range(N):
    if n[u] == -1:
        visit(u)

print(ans)