import sys sys.setrecursionlimit(10**6) input = sys.stdin.readline """ Strongly Connected Components source ; https://tjkendev.github.io/procon-library/python/graph/scc.html """ # 強連結成分分解(SCC): グラフGに対するSCCを行う # 入力: : 頂点サイズ, : 順方向の有向グラフ, : 逆方向の有向グラフ # 出力: (<ラベル数>, <各頂点のラベル番号>) def scc(N, G, RG): order = [] used = [0]*N group = [None]*N def dfs(s): used[s] = 1 for t in G[s]: if not used[t]: dfs(t) order.append(s) def rdfs(s, col): group[s] = col used[s] = 1 for t in RG[s]: if not used[t]: rdfs(t, col) for i in range(N): if not used[i]: dfs(i) used = [0]*N label = 0 for s in reversed(order): if not used[s]: rdfs(s, label) label += 1 return label, group # 縮約後のグラフを構築 def construct(N, G, label, group): G0 = [set() for i in range(label)] GP = [[] for i in range(label)] for v in range(N): lbs = group[v] for w in G[v]: lbt = group[w] if lbs == lbt: continue G0[lbs].add(lbt) GP[lbs].append(v) return G0, GP """ Main Code """ N,M = map(int,input().split()) G = [[] for _ in [0]*N] GR = [[] for _ in [0]*N] for _ in [0]*M: a,b = map(int,input().split()) G[a-1].append(b-1) GR[b-1].append(a-1) label,group = scc(N,G,GR) DAG,_ = construct(N,G,label,group) t = len(DAG) if(t == 1): print(0) exit(0) source = [True]*t sink = [True]*t for i in range(t): for j in DAG[i]: sink[i] = source[j] = False ans = max(sum(source),sum(sink)) print(ans)