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#define debug_printf(fmt, ...)
//#define debug_printf(fmt, ...) do { printf(fmt, __VA_ARGS__); } while (0)

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <climits>
#include <cassert>
#include <vector>
#include <set>
#include <map>
#include <deque>
#include <algorithm>
#include <functional>
#include <random>
#include <limits>
#include <iostream>

const long long LLMAX = (std::numeric_limits<long long>()).max();
const long long LLMIN = (std::numeric_limits<long long>()).min();

const int N_max = 500000;
const int M_max = 1000000;

struct Road {
    int a;
    int b;

    static bool less(const Road* p, const Road* q) {
        return
            (p->a != q->a)
            ? p->a < q->a
            : p->b < q->b;
    }
};

Road roads_buf[M_max];
Road* roads_tmp[M_max];

struct Crossroad {
    Road** roads;
    int roads_len;

    bool is_visited;
};

Crossroad crossroads[N_max + 1];   // 1-based

int crossroad_num(Crossroad& crossroad) {
    return &crossroad - crossroads;
}

bool cycle_found;
std::set<int> always_present;

void visit(Crossroad& crossroad, std::vector<int>& path, std::set<int>& in_path, std::set<int>& always_present) {
    crossroad.is_visited = true;
    path.push_back(crossroad_num(crossroad));
    in_path.insert(crossroad_num(crossroad));
    for (auto& cr : path) { debug_printf("%d ", cr); } debug_printf("\n");
    auto old_visited = std::find_if(
        crossroad.roads,
        crossroad.roads + crossroad.roads_len,
        [](const Road* r) { return crossroads[r->b].is_visited; }
    );
    if (old_visited == crossroad.roads + crossroad.roads_len) {
        for (int i=0; i < crossroad.roads_len; ++i) {
            Road& road = *crossroad.roads[i];
            Crossroad& other_crossroad = crossroads[road.b];
            assert(!other_crossroad.is_visited);
            visit(other_crossroad, path, in_path, always_present);
        }
    } else {
        cycle_found = true;

        for (auto it=always_present.begin(); it != always_present.end(); ) {
            Crossroad& cr = crossroads[*it];
            if (!cr.is_visited) {
                it = always_present.erase(it);
            } else {
                ++it;
            }
        }
    }
    crossroad.is_visited = false;
    path.pop_back();
    in_path.erase(crossroad_num(crossroad));
}

int main()
{
    std::ios_base::sync_with_stdio(false);

    int N, M; scanf("%d %d", &N, &M);

    for (int i=0; i < M; ++i) {
        int ai, bi; scanf("%d %d", &ai, &bi);
        Road& road = roads_buf[i];
        road.a = ai;
        road.b = bi;
        roads_tmp[i] = &road;
    }

    std::sort(roads_tmp, roads_tmp + M, Road::less);

    int curr_crossroad = -1;
    for (int i=0; i < M; ++i) {
        Road& road = *roads_tmp[i];
        if (curr_crossroad != road.a) {
            curr_crossroad = road.a;
            crossroads[curr_crossroad].roads = &roads_tmp[i];
        }
        ++crossroads[curr_crossroad].roads_len;
    }

    for (int i=1; i <= N; ++i) {
        always_present.insert(i);
    }
    std::vector<int> path;
    std::set<int> in_path;
    for (int i=1; i <= N; ++i) {
        Crossroad& crossroad = crossroads[i];
        in_path.clear();
        std::for_each(
            crossroads + 1,
            crossroads + N + 1,
            [](Crossroad& cr) { cr.is_visited = false; }
        );
        visit(crossroad, path, in_path, always_present);
    }

    if (cycle_found) {
        printf("%d\n", always_present.size());
        for (auto& cr : always_present) { printf("%d ", cr); };
        printf("\n");
    } else {
        printf("NIE\n");
    }
}