#include <iostream>
#include <vector>
#include <map>
using namespace std;

struct Graph
{
    int n;
    std::vector<std::vector<int>> g;
    
    Graph(){}
    
    Graph(int n) : n(n){
        g.resize(n);
    }
    
    void add_edge(int from, int to){
        g[from].push_back(to);
    }
};

struct SCC      //StronglyConnectedComponents
{
    int n;
    int k;
    std::vector<std::vector<int>> g, rg;
    std::vector<bool> used;
    std::vector<int> cmp;
    std::vector<int> vs;
    
    SCC(){}
    
    SCC(int n) : n(n){
        g.resize(n);
        rg.resize(n);
        used.resize(n);
        cmp.resize(n);
    }
    
    void add_edge(int from, int to){
        g[from].push_back(to);
        rg[to].push_back(from);
    }
    
    void dfs(int u){
        used[u] = true;
        for(int v : g[u]){
            if(!used[v]) dfs(v);
        }
        vs.push_back(u);
    }
    
    void rdfs(int u, int k){
        used[u] = true;
        cmp[u] = k;
        for(int v : rg[u]){
            if(!used[v]) rdfs(v, k);
        }
    }
    
    int decomposition(){
        for(int i = 0; i < n; i++) used[i] = false;
        for(int i = 0; i < n; i++){
            if(!used[i]) dfs(i);
        }
        for(int i = 0; i < n; i++) used[i] = false;
        k = 0;
        for(int i = n - 1; i >= 0; i--){
            if(!used[vs[i]]){
                rdfs(vs[i], k);
                k++;
            }
        }
        return k;
    }
    
    int decomposition(Graph &dag){
        k = decomposition();
        dag.n = k;
        dag.g.resize(k);
        std::map<std::pair<int, int>, bool> mp;
        for(int u = 0; u < n; u++){
            for(int v : g[u]){
                if(!mp[std::pair<int, int> (cmp[u], cmp[v])] && cmp[u] != cmp[v]){
                    mp[std::pair<int, int> (cmp[u], cmp[v])] = true;
                    dag.add_edge(cmp[v], cmp[u]);
                }
            }
        }
        return k;
    }
};

int main()
{
    int n, m;
    cin >> n >> m;
    SCC g(n);
    for(int i = 0; i < m; i++){
        int a, b;
        cin >> a >> b;
        a--; b--;
        g.add_edge(a, b);
    }
    Graph h;
    int k = g.decomposition(h);
    if(k == 1){
        cout << 0 << endl;
        return 0;
    }
    int d[100005]{0};
    for(int u = 0; u < k; u++){
        for(int v : h.g[u]) d[v]++;
    }
    int c0 = 0, c1 = 0;
    for(int u = 0; u < k; u++){
        if(h.g[u].size() == 0) c0++;
        if(d[u] == 0) c1++;
    }
    cout << max(c0, c1) << endl;
}