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#include <cstdio>
#include <vector>
#include <list>
#include <unordered_set>
#include <set>
#include <unordered_map>
#include <map>
#include <utility>
using namespace std;

namespace std {
    template <>
    struct hash<std::vector<int>> {
        size_t operator()(const vector<int>& v) const {
            std::hash<int> hasher;
            size_t seed = 0;
            for (int i : v) {
                seed ^= hasher(i) + 0x9e3779b9 + (seed<<6) + (seed>>2);
            }
            return seed;
        }
    };
}

void dfs(int v,
         vector<list<int> > &ve,
         vector<int> &base_vector,
         map<int, int> &base_map,
         vector<pair<vector<int>, map<int, int> > > &cycles,
         set<vector<int> > pureCycles,
         map<int, list<int> > &name,
         set<int> &visited) {
    if (visited.find(v) != visited.end()) {
        if (base_map.find(v) != base_map.end()) {
            vector<int> cycle_vector (base_vector.begin() + (*(base_map.find(v))).second, base_vector.end());
            cycle_vector.push_back(v);
            map<int, int> cycle_map;
            int i = 0;
            for (auto it = cycle_vector.begin(); it != cycle_vector.end(); ++it, ++i) {
                cycle_map.insert(pair<int, int> (*it, i));
                name[*it].push_back(cycles.size());
            }

            cycles.push_back(pair<vector<int>, map<int, int> > (cycle_vector, cycle_map));
            pureCycles.insert(cycle_vector);
        } else {
            list<vector<int> > new_cycles;
            for (auto it = name[v].begin(); it != name[v].end(); ++it) {
                //printf("cycle %d %d\n", name[v].size(), cycles.size());
                vector<int> cycle_vector = cycles[*it].first;
                map<int, int> cycle_map = cycles[*it].second;
                if (base_map.find(cycle_vector.back()) != base_map.end()) {
                    vector<int> new_cycle_vector (base_vector.begin() + (*(base_map.find(cycle_vector.back()))).second, base_vector.end());
                    new_cycle_vector.insert(new_cycle_vector.end(), cycle_vector.begin() + (*(cycle_map.find(v))).second, cycle_vector.end());

                    new_cycles.push_back(new_cycle_vector);
                } else { //puts("else");
                }
            }
            for (auto it = new_cycles.begin(); it != new_cycles.end(); ++it) {
                if (pureCycles.find(*it) == pureCycles.end()) {
                    map<int, int> new_cycle_map;
                    int i = 0;
                    for (auto it2 = (*it).begin(); it2 != (*it).end(); ++it2, ++i) {
                        new_cycle_map.insert(pair<int, int> (*it2, i));
                        name[*it2].push_back(cycles.size());
                    }

                    cycles.push_back(pair<vector<int>, map<int, int> > (*it, new_cycle_map));
                    pureCycles.insert(*it);
                }
            }
        }

//            for (auto it = cycles.begin(); it != cycles.end(); ++it) {
//                printf("%d", (*it).first.size());
//                puts("");
//            }

        return;
    }
    
    visited.insert(v);

    base_vector.push_back(v);
    base_map.insert(pair<int, int> (v, base_map.size()));

    for (auto it = ve[v].begin(); it != ve[v].end(); ++it) {
        dfs(*it, ve, base_vector, base_map, cycles, pureCycles, name, visited);
    }

    base_vector.pop_back();
    base_map.erase(v);
}

int main() {
    int n, m;
    scanf("%d %d", &n, &m);

    vector<list<int> > ve (n, list<int> ());

    for (int i = 0; i < m; ++i) {
        int a, b;
        scanf("%d %d", &a, &b);
        --a;
        --b;

        ve[a].push_back(b);
    }

    vector<int> base_vector;
    map<int, int> base_map;
    vector<pair<vector<int>, map<int, int> > > cycles;
    set<vector<int> > pureCycles;
    map<int, list<int> > name;
    set<int> visited;

    for (int i = 0; i < n && visited.size() < n; ++i) {
        if (visited.find(i) == visited.end()) {
            dfs(i, ve, base_vector, base_map, cycles, pureCycles, name, visited);
        }
    }


//    for (auto it = cycles.begin(); it != cycles.end(); ++it) {
//        for (auto it2 = ((*it).first).begin(); it2 != ((*it).first).end(); ++it2) {
//            printf("%d ", (*it2)+1);
//        }
//        puts("");
//    }


    if (cycles.size() > 0) {
        set<int> intersection (cycles.front().first.begin(), cycles.front().first.end());
        for (auto it = cycles.begin(); it != cycles.end(); ++it) {
            set<int> new_intersection;
            for (auto it2 = intersection.begin(); it2 != intersection.end(); ++it2) {
                if ((*it).second.find(*it2) != (*it).second.end()) {
                    new_intersection.insert(*it2);
                }
            }
            intersection = new_intersection;
        }

        printf("%d\n", intersection.size());
        for (auto it = intersection.begin(); it != intersection.end(); ++it) {
            printf("%d ", (*it) + 1);
        }
        printf("\n");
    } else {
        printf("NIE");
    }

    return 0;
}