/*

ID: LinKArftc

PROG: 2186.cpp

LANG: C++

*/

#include <map>

#include <set>

#include <cmath>

#include <stack>

#include <queue>

#include <vector>

#include <cstdio>

#include <string>

#include <utility>

#include <cstdlib>

#include <cstring>

#include <iostream>

#include <algorithm>

using namespace std;

#define eps 1e-8

#define randin srand((unsigned int)time(NULL))

#define input freopen("input.txt","r",stdin)

#define debug(s) cout << "s = " << s << endl;

#define outstars cout << "*************" << endl;

const double PI = acos(-1.0);

const int inf = 0x3f3f3f3f;

const int INF = 0x7fffffff;

typedef long long ll;

const int maxn = ;

const int maxm = ;

int n, m;

struct Edge {

    int v, next;

} edge[maxm];

int head[maxn], tot;

void init() {

    tot = ;

    memset(head, -, sizeof(head));

}

void addedge(int u, int v) {

    edge[tot].v = v;

    edge[tot].next = head[u];

    head[u] = tot ++;

}

int dfn[maxn], low[maxn], belong[maxn];//每个点属于哪个连通分量

int scc;//连通分量总数,下标 1~scc

int Time;

int ins[maxn];

stack <int> s;

vector <int> vec[maxn];//记录每个连通分量内的点

void tarjan(int u) {

    int v;

    low[u] = dfn[u] = ++ Time;

    s.push(u);

    ins[u] = true;

    for (int i = head[u]; i + ; i = edge[i].next) {

        v = edge[i].v;

        if (!dfn[v]) {

            tarjan(v);

            low[u] = min(low[u], low[v]);

        } else if (ins[v]) low[u] = min(low[u], dfn[v]);//写出low[u] = min(low[u], low[v]);也可以

    }

    if (low[u] == dfn[u]) {

        scc ++;

        do {

            v = s.top();

            s.pop();

            ins[v] = false;

            belong[v] = scc;

            vec[scc].push_back(v);

        } while (u != v);

    }

}

int outdeg[maxn];

int main() {

    //input;

    while (~scanf("%d %d", &n, &m)) {

        init();

        int u, v;

        for (int i = ; i <= m; i ++) {

            scanf("%d %d", &u, &v);

            addedge(u, v);

        }

        while (!s.empty()) s.pop();

        for (int i = ; i <= maxn; i ++) vec[i].clear();

        memset(dfn, , sizeof(dfn));

        memset(ins, , sizeof(ins));

        Time = ; scc = ;

        for (int i = ; i <= n; i ++) {

            if (!dfn[i]) tarjan(i);

        }

        memset(outdeg, , sizeof(outdeg));

        for (int u = ; u <= n; u ++) {

            for (int i = head[u]; i + ; i = edge[i].next) {

                int v = edge[i].v;

                if (belong[u] == belong[v]) continue;

                outdeg[belong[u]] ++;

            }

        }

        int ans = ;

        bool has = false;

        bool flag = false;

        int ii;

        for (int i = ; i <= scc; i ++) {

            if (outdeg[i] == ) {

                ii = i;

                if (has) {

                    flag = false;

                    break;

                } else {

                    has = true;

                    flag = true;

                }

            }

        }

        if (flag) printf("%d\n", vec[ii].size());

        else printf("0\n");

    }

    return ;

}