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#include <iostream>
#include <vector>
#include <unordered_set>
#include <algorithm>


struct Vertex {
    std::vector<int> next;
    int dfsnum, low, cycleIndex;
    bool T, inGraph, inCycle, finished, removedFromCycle;
    Vertex() : dfsnum(), low(), cycleIndex(-1), T(), inGraph(true), inCycle(), finished(false), removedFromCycle(false) {}
};

using Graph = std::vector<Vertex>;
using StronglyCC = std::vector<int>;
using StronglyCCs = std::vector<StronglyCC>;

void findStronglyConnectedComponents(Graph &graph, StronglyCCs &stronglyConnectedComponents);
void findCycle(Graph &graph, StronglyCC &component, std::vector<int> &cycle);
bool visit(Graph &graph, int p, std::vector<int> &cycle, int cycleIndex);

int nodesNotRemovedFromCycle = -1;

int main() {
    std::ios_base::sync_with_stdio(false);
    int n, m;
    std::cin >> n >> m;
    Graph graph;
    graph.resize(n + 1);
    graph[0].next.resize(n); // fake node
    for (int i = 0; i < n; ++i) {
        graph[0].next[i] = i + 1;
    }
    int b, e;
    for (int i = 0; i < m; ++i) {
        std::cin >> b >> e;
        graph[b].next.push_back(e);
    }
    StronglyCCs stronglyConnectedComponents;
    findStronglyConnectedComponents(graph, stronglyConnectedComponents);
    // there are at least two components because of fake node
    int maxComponentNumber = stronglyConnectedComponents[0].size() > stronglyConnectedComponents[1].size() ? 0 : 1;
    int secondMaxComponentNumber = 1 - maxComponentNumber;
    for (int i = 2; i < int(stronglyConnectedComponents.size()); ++i) {
        if (stronglyConnectedComponents[maxComponentNumber].size() < stronglyConnectedComponents[i].size()) {
            secondMaxComponentNumber = maxComponentNumber;
            maxComponentNumber = i;
        } else if (stronglyConnectedComponents[secondMaxComponentNumber].size() < stronglyConnectedComponents[i].size()) {
            secondMaxComponentNumber = i;
        }
    }
    if (stronglyConnectedComponents[maxComponentNumber].size() == 1) {
        std::cout << "NIE";
        return 0;
    }
    if (stronglyConnectedComponents[secondMaxComponentNumber].size() > 1) {
        std::cout << 0 << "\n";
        return 0;
    }
    // remove from graph everything except single component
    for (int i = 0; i < int(stronglyConnectedComponents.size()); ++i) {
        if (i != maxComponentNumber) {
            for (auto &v : stronglyConnectedComponents[i]) {
                graph[v].inGraph = false;
            }
        }
    }
    for (auto &v : graph) {
        if (v.inGraph) {
            v.next.erase(std::remove_if(v.next.begin(), v.next.end(), [graph](int v) {
                return !graph[v].inGraph;
            }), v.next.end());
        }
    }
    StronglyCC &component = stronglyConnectedComponents[maxComponentNumber];
    std::vector<int> cycle;
    for (auto v : component) { // reset dfs marker
        graph[v].T = false;
    }
    findCycle(graph, component, cycle);
    nodesNotRemovedFromCycle = cycle.size();
    for (int i = 0; i < int(cycle.size()); ++i) { // dfs from each node in cycle
        graph[cycle[i]].T = true;
        if (visit(graph, cycle[i], cycle, i)) { // answer contains 0 nodes
            std::cout << 0 << "\n";
            return 0;
        }
    }
    std::vector<int> answer;
    for (auto v : cycle) {
        if (!graph[v].removedFromCycle) {
            answer.push_back(v);
        }
    }
    std::sort(begin(answer), end(answer));
    std::cout << answer.size() << "\n";
    for (auto x : answer) {
        std::cout << x << " ";
    }
}

void visitSSC(Graph &graph, int p, StronglyCCs &stronglyConnectedComponents, int &N, std::vector<int> &L);

void findStronglyConnectedComponents(Graph &graph, StronglyCCs &stronglyConnectedComponents) {
    int N = 0;
    std::vector<int> L;
    L.reserve(graph.size());
    graph[0].T = true;
    visitSSC(graph, 0, stronglyConnectedComponents, N, L);
    for (auto &v : graph) {
        v.inGraph = true;
    }
}

void visitSSC(Graph &graph, int p, StronglyCCs &stronglyConnectedComponents, int &N, std::vector<int> &L) {
    L.push_back(p);
    graph[p].dfsnum = N++;
    graph[p].low = graph[p].dfsnum;
    for (auto &q : graph[p].next) {
        if (!graph[q].inGraph) {
            continue;
        }
        if (!graph[q].T) {
            graph[q].T = true;
            visitSSC(graph, q, stronglyConnectedComponents, N, L);
            graph[p].low = std::min(graph[p].low, graph[q].low);
        } else {
            graph[p].low = std::min(graph[p].low, graph[q].dfsnum);
        }
    }
    if (graph[p].low == graph[p].dfsnum) {
        stronglyConnectedComponents.emplace_back();
        int last = 0;
        do {
            last = L.back();
            L.pop_back();
            stronglyConnectedComponents.back().push_back(last);
            graph[last].inGraph = false;
        } while (last != p);
    }
}

void findCycle(Graph &graph, StronglyCC &component, std::vector<int> &cycle) {
    int current = component[0];
    while (!graph[current].T) {
        graph[current].T = true;
        current = graph[current].next[0];
    }
    int start = current;
    int index = 0;
    do {
        cycle.push_back(current);
        graph[current].inCycle = true;
        graph[current].cycleIndex = index++;
        current = graph[current].next[0];
    } while (current != start);
    current = component[0];
    while (graph[current].T) { // reset dfs marker
        graph[current].T = false;
        current = graph[current].next[0];
    }
}

int lastRemovalStart = 0, lastRemovalEnd = 1;
bool removeFromCycle(Graph &graph, std::vector<int> &cycle, int from, int to) {
    if (to <= from) {
        to += cycle.size();
    }
    int removeStart = graph[cycle[from]].removedFromCycle ? lastRemovalEnd - 1 : from;
    int removeStop = to < lastRemovalEnd ? lastRemovalEnd : to;
    if (removeStop - removeStart == 1) { // nothing to be done
        return false;
    }
    lastRemovalEnd = removeStop;
    int firstStop = std::min(removeStop, int(cycle.size()));
    for (int i = removeStart + 1; i < firstStop; ++i) {
        if (!graph[cycle[i]].removedFromCycle) {
            graph[cycle[i]].removedFromCycle = true;
            --nodesNotRemovedFromCycle;
        }
    }
    for (int i = 0; i < removeStop - int(cycle.size()); ++i) {
        if (!graph[cycle[i]].removedFromCycle) {
            graph[cycle[i]].removedFromCycle = true;
            --nodesNotRemovedFromCycle;
        }
    }
    return nodesNotRemovedFromCycle == 0;
}

bool visit(Graph &graph, int p, std::vector<int> &cycle, int cycleIndex) {
    for (int q : graph[p].next) {
        if (graph[q].inCycle) {
            if (removeFromCycle(graph, cycle, cycleIndex, graph[q].cycleIndex)) { // everything removed from cycle
                return true;
            }
            continue;
        } else if (graph[q].T) {
            if (!graph[q].finished) { // we have found second disjoint cycle
                return true;
            }
        }
        graph[q].T = true;
        if (visit(graph, q, cycle, cycleIndex)) {
            return true;
        }
    }
    graph[p].finished = true;
    return false;
}