import sys

# sys.setrecursionlimit(1000005)
# sys.set_int_max_str_digits(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
# inf = -1-(-1 << 31)
inf = -1-(-1 << 62)

# md = 10**9+7
md = 998244353

# 各強連結成分をリストにしてトポロジカル順に返す
def SCC(to, ot):
    n = len(to)
    # トポロジカルソート
    fin = [-1]*n
    topo = []
    for u in range(n):
        if fin[u] != -1: continue
        stack = [u]
        while stack:
            u = stack[-1]
            if fin[u] == -1:
                fin[u] = 0
                for v in to[u]:
                    if fin[v] != -1: continue
                    stack.append(v)
            else:
                stack.pop()
                if fin[u] == 0:
                    fin[u] = 1
                    topo.append(u)
    # 逆辺でdfs
    res = []
    while topo:
        u = topo.pop()
        if fin[u] != 1: continue
        fin[u] = 2
        cur = [u]
        i = 0
        while i < len(cur):
            u = cur[i]
            for v in ot[u]:
                if fin[v] == 2: continue
                fin[v] = 2
                cur.append(v)
            i += 1
        res.append(cur)

    return res

n,m=LI()
to=[[] for _ in range(n)]
ot=[[] for _ in range(n)]
for _ in range(m):
    u,v=LI1()
    to[u].append(v)
    ot[v].append(u)

gg=SCC(to,ot)
gn=len(gg)

if gn==1:
    print(0)
    exit()

u2g=[0]*n
for g,uu in enumerate(gg):
    for u in uu:
        u2g[u]=g

indeg=[0]*gn
outdeg=[0]*gn
for u in range(n):
    g=u2g[u]
    for v in to[u]:
        h=u2g[v]
        if g==h:continue
        outdeg[g]+=1
        indeg[h]+=1

print(max(indeg.count(0),outdeg.count(0)))