#include<cstdio>

#include<cstring>

#include<algorithm>

using namespace std;

const int N = 10003;

const int M = 50003;

void read(int &k) {

	k = 0; int fh = 1; char c = getchar();

	for(; c < '0' || c > '9'; c = getchar())

		if (c == '-') fh = -1;

	for(; c >= '0' && c <= '9'; c = getchar())

		k = (k << 1) + (k << 3) + c - '0';

	k = k * fh;

}

struct node {

	int from, nxt, to;

}E[M];

bool vis[N], inst[N];

int point[N], ans = 0, color = 0, col[N], in[N], out[N], sum[N], cnt = 0, n, m, DFN[N], low[N], st[N], top = 0;

void ins(int x, int y) {E[++cnt] = (node) {x, point[x], y}; point[x] = cnt;}

void tarjan(int x) {

	DFN[x] = low[x] = ++cnt;

	st[++top] = x; inst[x] = vis[x] = 1;

	for(int tmp = point[x]; tmp; tmp = E[tmp].nxt)

		if (!vis[E[tmp].to])

			tarjan(E[tmp].to), low[x] = min(low[x], low[E[tmp].to]);

		else if (inst[E[tmp].to]) low[x] = min(low[x], DFN[E[tmp].to]);

	if (DFN[x] == low[x]) {

		++color;

		int u = 0;

		while (u != x) {

			u = st[top--];

			col[u] = color;

			inst[u] = 0;

			++sum[color];

		}

	}

}

int main() {

	read(n); read(m);

	int u, v;

	for(int i = 1; i <= m; ++i) {read(u); read(v); ins(u, v);}

	cnt = 0;

	tarjan(1);

	for(int i = 1; i <= m; ++i) {

		u = col[E[i].from]; v = col[E[i].to];

		if (u != v) ++out[u];

	}

	for(int i = 1; i <= color; ++i)

		if (out[i] == 0) {

			if (ans == 0) ans = sum[i];

			else {ans = 0; break;}

		}

	printf("%d\n", ans);

	return 0;

}