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#include <algorithm>
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <string.h>
using namespace std;

vector<vector<int>>G;
int n, m;
int *V;


bool dfs(int start, int excl) {
	vector<int>S;
	S.push_back(start);
	while (!S.empty()){
		int p = S.back();
		if (V[p] == 0) {
			V[p] = 1;
			for (int q : G[p]) {
				if (V[q] == 1) {
					return true;
				}
				else if (!V[q] && q != excl) {
					S.push_back(q);
				}
			}
		}
		else {
			V[p] = 2;
			S.pop_back();
		}
	}
	return false;
}

int solveBrut() {
	int res = 0;
	for (int excl = 0; excl < n; excl++) {
		memset(V, 0, n*sizeof(int));
		bool found = false;
		for (int start = 0; start < n; start++){
			if (V[start] == 0 && start != excl) {
				found |= dfs(start, excl);
			}
		}
		if (!found) {
			//cout << excl + 1 << " on every\n";
			res++;
		}
	}
	return res;
}



int dfsCount(int start,int offset, vector<int>& backOpen) {
	vector<int>S;
	vector<int> count;
	vector<int> index;
	vector<int> open;
	vector<int> backEnd;

	index.resize(n);
	open.resize(n);
	count.resize(n);
	backEnd.resize(n);
	S.push_back(start);
	while (!S.empty()){
		int p = S.back();
		if (V[p] == 0) {
			count[p] = 0;
			V[p] = 1;
			open[p] = -1;
			index[p] == 0;
			backEnd[p] = 0;
			backOpen[p] = 0;
		}
		else {
			if (open[p] != -1) {
				int q = open[p];
				count[p] += count[q];
				index[p] = index[p] + 1;
				backOpen[p] += backOpen[q] - backEnd[q];
				open[p] = -1;
			}
			else {
				if (index[p] == G[p].size())  {
					V[p] = 2;
					S.pop_back();
					continue;
				}
				else {
					int edges = G[p].size();
					int q = G[p][(index[p]+start)%edges];
					if (V[q] == 0) {
					//	cout << "drzewna: " << p + 1 << " " << q + 1 << "\n";
						S.push_back(q);
						open[p] = q;
					}
					else if (V[q] == 1) {
					//	cout << "wsteczna " << p + 1 << " " << q + 1 << "\n";
						count[p]++;
						index[p]++;
						backEnd[q] += 1;
						backOpen[p] += 1;
					}
					else
					{
					//	cout << "poprzeczna: " << p + 1 << " " << q + 1 << "\n";
						index[p]++;
					}
				}
			}
		}
	}
	return count[start];
}

bool *candidate;

int findCand(int offset) {
	memset(V, 0, n*sizeof(int));
	vector<int> open;
	open.resize(n,0);
	int count = 0;

	for (int start = 0; start < n; start++){
		int p = (offset + start) % n;
		if (V[p] == 0) {
			count += dfsCount(p, offset,open);
		}
	}

	if (count == 0) {
		cout << "NIE\n";
		return -1;
	}
	vector<int> candidates;
	for (int i = 0; i < n; i++) {
		if (open[i] != count) candidate[i] = false;
	}
}

int solve() {
	candidate = new bool[n];
	for (int i = 0; i < n; i++)candidate[i] = true;
	int runs = 0;
	for (int i = 0; i < n; i++) {
		if (!candidate[i])continue;
		int r = findCand(i);
		if (++runs == 75)break;
		if (r == -1)return -1;
	}
	int res = 0;
	for (int i = 0; i < n; i++)if (candidate[i])res++;
	cout << res << "\n";
	for (int i = 0; i < n;i++) {
		if(candidate[i])cout << i+1 << " ";
	}
	cout << "\n";
	return res;
}

void testFiles(){
	for (int i = 3; i <= 3; i++) {
		ifstream file;
		file.open("Medium/kon" + to_string(i) + ".in");
		ifstream outfile;
		outfile.open("Medium/kon" + to_string(i) + ".out");
		file >> n >> m;
		G.clear();
		G.resize(n);
		V = new int[n];
		for (int i = 0; i < m; i++) {
			int a, b;
			file >> a >> b;
			a--; b--;
			G[a].push_back(b);
		}

		cout << i << ":\n";
		//int res = solveBrut();
		int res = solve();
		int outres=0;
		outfile >> outres;
		cout <<"correct: " << outres << "\n";
		file.close();
	}
}

int main()
{
	std::ios_base::sync_with_stdio(false);
	cin >> n >> m;
	G.resize(n);
	V = new int[n];
	for (int i = 0; i < m; i++) {
		int a, b;
		cin >> a >> b;
		a--; b--;
		//a=i;b=(i+1)%n;
		G[a].push_back(b);
	}
	int res =solve();
//	testFiles();
	return 0;
}