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#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
#include <array>

using std::begin;
using std::end;

constexpr uint32_t out_of_value = 2000001;

class node {
public:
  static uint32_t visit_part;
  static uint32_t numerator;
  uint32_t visit_numerator = 0;
  uint32_t my_end = out_of_value;
  std::vector<size_t> neightbours, reversed;
  uint32_t dfs2(const uint32_t);
  public:
   //bool in_cycle = false;
   int32_t number = -2;
   static void next_visit_part();
   inline bool visited();
   void connect(const size_t);
   void post_order();
   void dfs1();
   node* find_cycle();
   void clear();
   void find_end();
   bool other_cycle();
 } *graph;

uint32_t node::visit_part = 0; 
uint32_t node::numerator = 0;
std::vector<std::vector<size_t>> scc;
std::vector<size_t> cycle;

int main() {
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(NULL);
  uint32_t n, m;
  std::cin >> n >> m;
  graph = new node[n];
  for(uint32_t a, b, i = 0; i < m; ++i) {
    std::cin >> a >> b;
    graph[a-1].connect(b-1);
   }
  node::next_visit_part();
  for(size_t i = 0; i < n; ++i) {
    if(graph[i].visited() == false) {
      graph[i].post_order();
     }
   }
  size_t* const sorted = new size_t[n];
  for(size_t i = 0; i < n; ++i) {
    sorted[graph[i].number] = i;
   }
  std::reverse(sorted, sorted+n);
  node::next_visit_part();
  for(size_t i = 0; i < n; ++i) {
    scc.emplace_back();
    if(graph[sorted[i]].visited() == false) {
      graph[sorted[i]].dfs1();
     }
    if(scc.back().size() <= 1) {
      scc.pop_back();
     }
   }
  if(scc.empty() == true) {
    std::cout << "NIE\n";
    return 0;
   }
  if(scc.size() > 1) {
    std::cout << 0 << '\n' << '\n';
    return 0;
   }
  for(const auto& x : scc.back()) {
    graph[x].number = -3;
   }
  for(const auto& x : scc.back()) {
    graph[x].clear();
   }
  node::next_visit_part();
  graph[scc.back().front()].find_cycle();
  std::reverse(begin(cycle), end(cycle));
  for(const auto x : scc.back()) {
    if(graph[x].other_cycle() == true) {
      std::cout << 0 << '\n';
     }
   }
  node::next_visit_part();
  for(uint32_t i = 0; i < cycle.size(); ++i) {
    graph[cycle[i]].number = i;
   }
  for(size_t i = 0; i < cycle.size(); ++i) {
    graph[cycle[i]].find_end();
   }

  uint32_t temp_end = out_of_value;
  if(graph[cycle.front()].my_end != out_of_value) {
    temp_end = graph[cycle.front()].my_end;
   }
  bool correct = true;
  std::vector<bool> odpowiedzi;
  for(size_t i = 0; i < cycle.size(); ++i) {
    if(temp_end != out_of_value && static_cast<int32_t>(temp_end) == graph[cycle[i]].number) {
      temp_end = out_of_value;
      correct = true;
     }
    odpowiedzi.push_back(correct);
    if(graph[i].my_end != out_of_value) {
      if(temp_end == out_of_value) {
        temp_end = graph[i].my_end;
       }
      else if (temp_end <= i && graph[i].my_end <= i) {
        temp_end = std::max(temp_end, graph[i].my_end);
       }
      else if(temp_end > i && graph[i].my_end <= i) {
        temp_end = graph[i].my_end;
       }
      else {
        temp_end = std::max(temp_end, graph[i].my_end);
       }
     }
   if(temp_end != out_of_value) {
     correct = false;
    } else correct = true;
   }
  std::vector<uint32_t> answers;
  if(temp_end < out_of_value) {
    for(size_t i = 0; i < odpowiedzi.size() && cycle[i] != temp_end; ++i) {
      odpowiedzi[i] = false;
     }
   }
  for(size_t i = 0; i < odpowiedzi.size(); ++i) {
    if(odpowiedzi[i] == true) {
      answers.push_back(cycle[i]+1);
     }
   }
  std::sort(begin(answers), end(answers));
  std::cout << answers.size() << '\n';
  std::copy(begin(answers), end(answers), std::ostream_iterator<uint32_t>(std::cout, " "));
  std::cout << '\n';
 }

void node::connect(const size_t i) {
  neightbours.push_back(i);
  graph[i].reversed.push_back(this-graph);
 }

void node::post_order() {
  visit_numerator = visit_part;
  for(const auto& x : neightbours) {
    if(graph[x].visited() == false) {
      graph[x].post_order();
     }
   }
  number = numerator;
  numerator += 1;
 }

inline bool node::visited() {
  return visit_numerator == visit_part;
 }

void node::next_visit_part() {
  visit_part += 1;
 }

void node::dfs1() {
  visit_numerator = visit_part;
  for(const auto& x : reversed) {
    if(graph[x].visited() == false) {
      graph[x].dfs1();
     }
   }
  scc.back().push_back(this-graph);
 }

std::array<uint32_t, 2> out_of_value_element = {{out_of_value, out_of_value}};

uint32_t my_max(const uint32_t a, const uint32_t b, const uint32_t i) {
  if(a == out_of_value || b == out_of_value) {
    return std::min(a, b);
   }
  if(a <= i && b <= i) {
    return std::max(a,b);
   }
  if(a <= i || b <= i) {
    return std::min(a, b);
   }
  return std::max(a,b);
 }

uint32_t node::dfs2(const uint32_t first) {
  if(number >= 0) {
    return number;
   }
  uint32_t temp = out_of_value;
  visit_numerator = visit_part;
  for(const auto& x : neightbours) {
    if(graph[x].visit_numerator != visit_part) {
      temp = my_max(graph[x].dfs2(first), temp, first);
     }
   }
  return temp;
 }

void node::find_end() {
  next_visit_part();
  for(const auto& x : neightbours) {
    if(graph[x].number < 0 || graph[x].number != (number+1)%static_cast<int32_t>(cycle.size())) {
      my_end = my_max(graph[x].dfs2(number), my_end, number);
     }
   }
 }

node* node::find_cycle() {
  if(number < -3) {
    cycle.push_back(this - graph);
    return this;
   }
  number = -4;
  for(const auto& x : neightbours) {
    if(graph[x].number <= -3) {
      const auto temp = graph[x].find_cycle();
      if(temp == this || temp == nullptr) {
        return nullptr;
       }
      cycle.push_back(this - graph);
      return temp;
     }
   }
  return nullptr;
 }


void node::clear() {
  auto it = std::remove_if(begin(neightbours), end(neightbours),
                           [](const size_t i){return graph[i].number > -3;});
  neightbours.resize(std::distance(begin(neightbours), it));
 }

bool node::other_cycle() {
  if(visited() == true) {
    return true;
   }
  for(const auto& x : neightbours) {
    if(graph[x].number == -3) {
      if(graph[x].other_cycle() == true) {
        return true;
       }
     }
   }
  return false;
 }