import sys
input = sys.stdin.readline

"""
強連結成分分解（SCC）
ref : https://manabitimes.jp/math/1250
"""

class SCC:
    def __init__(self, n):
        self.n = n
        self.route = [[] for _ in [0] * n]
        self.rev_route = [[] for _ in [0] * n]
        self.label = [-1] * n
        self.count = 0
    
    def add_edge(self, v_first, v_second):
        self.route[v_first].append(v_second)
        self.rev_route[v_second].append(v_first)

    def _build(self):
        self.v_list = []
        self.visited = set([])
        for v in range(self.n):
            if(v in self.visited):
                continue
            self._dfs(v)
        for v in reversed(self.v_list):
            if(self.label[v] != -1):
                continue
            self._rev_dfs(v)
            self.count += 1

    def _dfs(self, v_start):
        stack = [v_start, 0]
        while(stack):
            v, id = stack[-2:]
            if(id == 0 and v in self.visited):
                stack.pop()
                stack.pop()
                continue
            self.visited.add(v)
            if(id < len(self.route[v])):
                stack[-1] += 1
                stack.extend([self.route[v][id], 0])
                continue
            stack.pop()
            self.v_list.append(stack.pop())

    def _rev_dfs(self, v_start):
        stack = [v_start]
        self.label[v_start] = self.count
        while(stack):
            v_now = stack.pop()
            for v_next in self.rev_route[v_now]:
                if(self.label[v_next] != -1):
                    continue
                self.label[v_next] = self.count
                stack.append(v_next)

    def construct(self):
        self._build()
        dag = [[] for _ in [0] * self.count]
        group = [[] for _ in [0] * self.count]
        for v, lb in enumerate(self.label):
            for v_next in self.route[v]:
                lb_next = self.label[v_next]
                if(lb == lb_next):
                    continue
                dag[lb].append(lb_next)
            group[lb].append(v)
        return dag, group

'''
Main Code
'''

n, m = map(int, input().split())
edges = [[x - 1 for x in map(int, input().split())] for _ in [0] * m]

scc = SCC(n)
for x, y in edges:
    scc.add_edge(x, y)
dag, _ = scc.construct()

N = len(dag)
if(N == 1):
    exit(print(0))
zero_in = [True] * N
zero_out = [True] * N
for i in range(N):
    if(dag[i]):
        zero_out[i] = False
    for j in dag[i]:
        zero_in[j] = False
ans = max(sum(zero_in), sum(zero_out))
print(ans)