#include<iostream>
#include<vector>
#include<cstring>
#include<algorithm>
#include<stack>
#define MAX_N 20002
using namespace std;

vector<int> G[MAX_N];
int dfn[MAX_N],low[MAX_N],ind=0;
bool vis[MAX_N];
bool inStack[MAX_N];

stack<int> st;

int color[MAX_N],tot=0;

vector<int> newG[MAX_N];
vector<int> newrG[MAX_N];

void Tarjan(int u) {
    dfn[u] = low[u] = ++ind;
    vis[u] = 1;
    inStack[u] = 1;
    st.push(u);

    for (int i = 0; i < G[u].size(); i++) {
        int v = G[u][i];
        if (!vis[v]) {
            Tarjan(v);
            low[u] = min(low[u], low[v]);
        }
        else if (inStack[v])
            low[u] = min(dfn[v], low[u]);
    }
    if (dfn[u] == low[u]) {
        int x;
        do {
            x = st.top();
            st.pop();
            inStack[x] = 0;
            color[x] = tot;
        } while (x != u);
        tot++;
    }
}

int main() {
    int n, m;
    cin.sync_with_stdio(false);
    while (cin >> n >> m) {
        memset(vis, 0, sizeof(vis));
        memset(inStack, 0, sizeof(inStack));
        while (st.size())st.pop();
        for (int i = 0; i <= n; i++)G[i].clear();
        for (int i = 0; i <= n; i++)newG[i].clear();
        for (int i = 0; i <= n; i++)newrG[i].clear();
        ind = 0;
        tot = 0;
        memset(color, 0, sizeof(color));

        for (int i = 0; i < m; i++) {
            int u, v;
            cin >> u >> v;
            G[u].push_back(v);
        }
        for (int i = 1; i <= n; i++)
            if (!vis[i])Tarjan(i);
        for (int i = 1; i <= n; i++)
            for (int j = 0; j < G[i].size(); j++) {
                int u = color[i], v = color[G[i][j]];
                if (u == v)continue;
                newG[u].push_back(v);
                newrG[v].push_back(u);
            }
        if (tot == 1) {
            cout << 0 << endl;
            continue;
        }
        int a = 0, b = 0;
        for (int i = 0; i < tot; i++)if (newG[i].size() == 0)a++;
        for (int i = 0; i < tot; i++)if (newrG[i].size() == 0)b++;
        cout << max(a, b) << endl;
    }
    return 0;
}