import sys

int1 = lambda x: int(x) - 1

# input = lambda: sys.stdin.buffer.readline()
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
i1 = lambda: int1(input())
mi = lambda: map(int, input().split())
mi1 = lambda: map(int1, input().split())
li = lambda: list(mi())
li1 = lambda: list(mi1())
lli = lambda n: [li() for _ in range(n)]

INF = float("inf")
mod = int(1e9 + 7)
# mod = 998244353

n, m = mi()
g = [list() for i in range(n)]
rg = [list() for i in range(n)]
for i in range(m):
    a, b = mi1()
    g[a].append(b)
    rg[b].append(a)

st = []
order = []
used = [False] * n
for i in range(n):
    st.append(i)
    while st:
        cur = st.pop()
        if cur < 0:
            order.append(-cur - 1)
            continue
        if used[cur]:
            continue
        used[cur] = True
        st.append(-cur - 1)
        for to in g[cur]:
            st.append(to)
cmp = [-1] * n
c = 0
for v in order[::-1]:
    if cmp[v] != -1:
        continue
    st.append(v)
    while st:
        cur = st.pop()
        if cmp[cur] != -1:
            continue
        cmp[cur] = c
        for to in rg[cur]:
            st.append(to)
    c += 1

IN = [0] * c
OUT = [0] * c
for cur in range(n):
    for to in g[cur]:
        if cmp[to] != cmp[cur]:
            OUT[cmp[cur]] += 1
            IN[cmp[to]] += 1

if c == 1:
    print(0)
else:
    print(max(IN.count(0), OUT.count(0)))