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#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,b) for (int i = (a); i <= (b); ++i)
#define REPD(i,a,b) for (int i = (a); i >= (b); --i)
#define FORI(i,n) REP(i,1,n)
#define FOR(i,n) REP(i,0,int(n)-1)
#define mp make_pair
#define pb push_back
#define pii pair<int,int>
#define vi vector<int>
#define ll long long
#define SZ(x) int((x).size())
#define DBG(v) cerr << #v << " = " << (v) << endl;
#define FOREACH(i,t) for (typeof(t.begin()) i=t.begin(); i!=t.end(); i++)
#define fi first
#define se second

const int maxn = 500500;
int n,m;
vi g[maxn], gg[maxn];
int vis[maxn], par[maxn];
int depth[maxn];

bool fndcycle; // czy znalazl
int fcyc, lcyc;
int cyc[maxn]; // cyc[fcyc..lcyc] - czesc wspolna
bool incycle[maxn]; // jest jest w ktorymkolwiek
int T;
int intime[maxn], outtime[maxn]; // czas wejscia i wyjscia

bool onpath(int low, int high, int u) {
	if (intime[u] < intime[high]) return false;
	if (intime[u] > intime[low]) return false;
	if (outtime[u] > outtime[high]) return false;
	if (outtime[u] < outtime[low]) return false;
	return true;
}

void addcyc(int u, int v) {
	if (!fndcycle) {
		fndcycle = true;
		int cur = u;
		incycle[u] = true;
		fcyc=0;
		lcyc=0;
		cyc[0]=u;
		while (cur != v) {
			cur = par[cur];
			incycle[cur] = true;
			cyc[++lcyc] = cur;
		}
	} else {
		while (fcyc<=lcyc && !onpath(u, v, cyc[fcyc])) {
			fcyc++;
		}
		while (fcyc<=lcyc && !onpath(u, v, cyc[lcyc])) {
			lcyc--;
		}
		if (fcyc>lcyc) {
			printf("0\n");
			exit(0);
		}
		int cur = u;
		while (cur != cyc[fcyc]) {
			incycle[cur] = true;
			cur = par[cur];
		}
		cur = cyc[lcyc];
		while (cur != v) {
			cur = par[cur];
			incycle[cur] = true;
		}
	}
}

void dfscyc(int u) {
	vis[u] = 1;
	intime[u]=++T;
	FOR(i,SZ(g[u])) {
		int v = g[u][i];
		if (vis[v] == 0) {
			par[v] = u;
			depth[v] = depth[u]+1;
			dfscyc(v);
		} else if (vis[v] == 1) {
			addcyc(u, v);
		}
	}
	vis[u] = 2;
	outtime[u]=++T;
}

bool nused[maxn];
bool checkcyc(int u) {
	vis[u]=1;
	FOR(i,SZ(g[u])) {
		int v=g[u][i];
		if (nused[v]) continue;
		if (vis[v]==1) return true;
		if (vis[v]==0) if (checkcyc(v)) return true;
	}
	vis[u]=2;
	return false;
}

int maxd[maxn], mind[maxn]; // max i min glebokosc
bool rev;

void dfsmax(int u) {
	vis[u] = 1;
	FOR(i,SZ(g[u])) {
		int v = g[u][i];
		if (!vis[v] && !incycle[v]) {
			dfsmax(v);
		}
		if (rev) mind[u] = min(mind[u], mind[v]);
		else maxd[u] = max(maxd[u], maxd[v]);
	}
}

vector<pii> rm;
void rmv(int left, int right) {
	left++;
	right--;
	if (right < left) return;
	//printf("usuwam %d %d\n", left, right);
	rm.pb(mp(left,right));
}

void rmvedge(int u, int v) {
	FOR(i,SZ(g[u])) if (g[u][i]==v) {
		swap(g[u][i], g[u][SZ(g[u])-1]);
		g[u].pop_back();
		return;
	}
	printf("nie znalazlem :(\n");
}

int main () {
	scanf("%d%d", &n, &m);
	FOR(i,m) {
		int a,b;
		scanf("%d%d", &a, &b);
		a--; b--;
		g[a].pb(b);
	}
	/*
	FOR(i,n+1) {
		printf("%d: ", i);
		FOREACH(j,g[i]) printf("%d ", *j);
		printf("\n");
	}
	*/
	fndcycle = false;
	FOR(i,n) if (vis[i] == 0) {
		par[i] = -1;
		dfscyc(i);
	}
	if (!fndcycle) {
		printf("NIE\n");
		return 0;
	}
	//printf("glebokosci: "); FOR(i,n) printf("%d ", depth[i]); printf("\n");
	//printf("mam kandydatow "); REP(i,fcyc,lcyc) printf("%d ", cyc[i]); printf("\n");
	fndcycle = false;
	REP(i,fcyc,lcyc) nused[cyc[i]]=true;
	FOR(i,n) vis[i]=0;
	FOR(i,n) if (!nused[i] && !vis[i]) {
		if (checkcyc(i)) {
			printf("0\n");
			return 0;
		}
	}
	REP(i,fcyc,lcyc-1) rmvedge(cyc[i+1], cyc[i]);
	FOR(i,n) vis[i]=0;
	FOR(i,n) {
		if (incycle[i]) maxd[i]=mind[i]=depth[i];
		else {
			maxd[i]=-1;
			mind[i]=n+5;
		}
	}
	REPD(i,lcyc,fcyc) {
		dfsmax(cyc[i]);
		//printf("max depth %d = %d\n", cyc[i], maxd[cyc[i]]);
		rmv(depth[cyc[i]], maxd[cyc[i]]);
	}
	rev=true;
	FOR(i,n) vis[i]=0;
	FOR(i,n) gg[i]=g[i];
	FOR(i,n) g[i].clear();
	FOR(i,n) FOR(j,SZ(gg[i])) g[gg[i][j]].pb(i);
	REP(i,fcyc,lcyc) {
		dfsmax(cyc[i]);
		//printf("min depth %d = %d\n", cyc[i], mind[cyc[i]]);
		rmv(mind[cyc[i]], depth[cyc[i]]);
	}
	rm.pb(mp(depth[cyc[fcyc]]+1,depth[cyc[fcyc]]+1));
	sort(rm.begin(), rm.end());
	int a=depth[cyc[lcyc]], b=0;
	int last = -1;
	vi res;
	//printf("rm: "); FOR(i,SZ(rm)) printf("[%d %d] ", rm[i].fi, rm[i].se); printf("\n");
	while (a <= depth[cyc[fcyc]] && b < SZ(rm)) {
		//printf("a=%d, b=%d (%d %d)\n", a, b, rm[b].fi, rm[b].se);
		if (rm[b].fi <= last) {
			last = max(rm[b].se, last);
			b++;
			continue;
		}
		if (a <= last) a = last+1;
		if (rm[b].fi <= a) {
			last = rm[b].se;
			b++;
			continue;
		}
		if (a > depth[cyc[fcyc]]) break;
		res.pb(a);
		a++;
	}
	sort(res.begin(), res.end());
	for (int i = lcyc, j = 0; i >= fcyc && j < SZ(res); ) {
		if (depth[cyc[i]]==res[j]) {
			res[j]=cyc[i];
			i--; j++;
		} else if (depth[cyc[i]] < res[j]) {
			i--;
		} else {
			printf("nie powinienem tu byc!\n");
		}
	}
	sort(res.begin(), res.end());
	printf("%d\n", SZ(res));
	FOR(i,res.size()) printf("%d ", res[i]+1);
	printf("\n");
	return 0;
}