#include <stdio.h>
#include <assert.h>
#include <queue>
#include <iostream>
#include <algorithm>
using namespace std;

#define all(a) a.begin(), a.end()
const int N = 1003;
int n, m; bool c[N][N];
vector<int> a1[N], a2[N], a[N];

namespace kosaraju {
    vector<int> Finish;
    int Dfs[N], Bfs[N];

    void dfs(int u) {
        Dfs[u]=true;
        for (int v: a1[u])
        if (!Dfs[v]) dfs(v);
        Finish.push_back(u);
    }

    void bfs(int S) {
        queue<int> qu;
        qu.push(S); Bfs[S]=S;
        while (qu.size()) {
            int u=qu.front(); qu.pop();
            for (int v: a2[u]) if (!Bfs[v])
            { qu.push(v); Bfs[v]=S; }
        }
    }

    void execute() {
        for (int i=1; i<=n; i++)
        if (!Dfs[i]) dfs(i);
        reverse(all(Finish));
        for (int u: Finish)
        if (!Bfs[u]) bfs(u);

        for (int u=1; u<=n; u++)
        for (int v: a1[u])
        c[Bfs[u]][Bfs[v]] = true;
    }
};

int Visited[N], t;
int Assigned[N];

bool visit(int u) {
    if (Visited[u]!=t) Visited[u]=t; else return false;
    for (int v: a[u])
    if (!Assigned[v] || visit(Assigned[v]))
    { Assigned[v]=u; return true; }
    return false;
}

int Dfs[N], S;

void dfs(int u) {
    if (Dfs[u]!=S) Dfs[u]=S; else return;
    if (u!=S) a[S].push_back(u);
    for (int i=1; i<=n; i++)
    if (c[u][i]) dfs(i);
}

main() {
    scanf("%d%d", &n, &m);
    for (int i=1; i<=m; i++) {
        int x, y; scanf("%d%d", &x, &y);
        a1[x].push_back(y);
        a2[y].push_back(x);
    }
    kosaraju :: execute();

    for (int i=1; i<=n; i++)
    if (kosaraju::Bfs[i]==i)
    { S=i; dfs(i); }

    int Result=0, Count=0;
    for (int i=1; i<=n; i++)
    if (kosaraju::Bfs[i]==i)
    { t++; Result+=!visit(i); }

    /*for (int i=1; i<=n; i++)
    if (kosaraju::Bfs[i]==i)
    printf("Assigned[%d] = %d\n", i, Assigned[i]);*/

    cout << Result << endl;
}