#include <iostream>
#include <vector>

using namespace std;

class My_Graph {
public:
    My_Graph(int V, vector<vector<int>> g, vector<vector<int>> t) {
        graph.resize(V);
        trans.resize(V);
        used.resize(V, false);
        help.resize(V);

        for (int i = 0; i < V; i++) {
            graph[i] = g[i];
            trans[i] = t[i];
        }

    }
    ~My_Graph() = default;

    void DFS_1(int vertex);
    void DFS_2(int vertex);
    void Build_Comp();
    int Get_Result();
private:
    vector<vector<int>> graph;
    vector<vector<int>> trans;
    vector<bool> used;
    vector<int> order;
    vector<int> help;
    vector<vector<int>> components;
    vector<int> component;
};

void My_Graph::DFS_1(int vertex) {
    used[vertex] = true;
    for (int i : graph[vertex]) {
        if (!used[i]) {
            DFS_1(i);
        }
    }
    order.push_back(vertex);
}

void My_Graph::DFS_2(int vertex) {
    used[vertex] = true;
    component.push_back(vertex);
    for (int i : trans[vertex]) {
        if (!used[i]) {
            DFS_2(i);
        }
    }
}

void My_Graph::Build_Comp() {
    used.assign(graph.size(), false);
    for (int i = 0; i < graph.size(); i++) {
        if (!used[i]) {
            DFS_1(i);
        }
    }
    used.assign(graph.size(), false);
    for (int i = 0; i < graph.size(); i++) {
        int temp = order[graph.size() - 1 - i];
        if (!used[temp]) {
            DFS_2(temp);
            for (int j : component) {
                help[j] = components.size();
            }
            components.push_back(component);
            component.clear();
        }
    }
}

int My_Graph::Get_Result() {
    Build_Comp();
    int begs = 0;
    int ends = 0;
    bool fl = true;

    for (int i = 0; i < components.size(); i++) {
        for (int v : components[i]) {
            for (int j : graph[v]) {
                if (help[j] != i) {
                    fl = false;
                }
            }
        }
        if (fl) begs++;
        fl = true;
    }

    for (int i = 0; i < components.size(); i++) {
        for (int v : components[i]) {
            for (int j : trans[v]) {
                if (help[j] != i) {
                    fl = false;
                }
            }
        }

        if (fl) ends++;
        fl = true;

    }



    int res = max(begs, ends);
    if (components.size() == 1) {
        res = 0;
    }
    return res;
}

int main() {
    int V = 0;
    int E = 0;
    cin >> V >> E;

    vector<vector<int>> graph;
    vector<vector<int>> trans;
    graph.resize(V);
    trans.resize(V);

    int u = 0;
    int v = 0;
    for (int i = 0; i < E; i++) {
        cin >> u >> v;
        graph[u - 1].push_back(v - 1);
        trans[v - 1].push_back(u - 1);
    }

    My_Graph Graph(V, graph, trans);
    cout << Graph.Get_Result();
    return 0;
}