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#include <cstdio>
#include <vector>
#include <queue>

const int MAX_N = 590009;
const int MAX_M = 1000009;

int n, m;
std::vector<int> adj[MAX_N];
std::vector<int> radj[MAX_N];
std::vector<int> s;
bool is_needed[MAX_N];
bool v[MAX_N];

void dfs(int a) {
  v[a] = true;
  for (int b : adj[a]) {
    if (!v[b]) {
      dfs(b);
    }
  }
  s.push_back(a);
}

int t;
int scc_no;
int scc[MAX_N];
std::pair<int, int> d[MAX_N];
std::deque<int> lower;
std::deque<int> higher;
std::deque<int> needed;
int path_no;
bool first_cycle;

void rdfs(int a) {
  d[a].first = ++t;
  scc[a] = scc_no;
  if (first_cycle)
    higher.push_back(a);
  for (int b : radj[a]) {
    if (scc[b] && scc[b] != scc_no)
      continue;
    if (!d[b].first) {
      // tree edge
      //fprintf(stderr, "tree\t%d -> %d\n", a, b);
      rdfs(b);
    } else if (!d[b].second) {
      // backward edge
      //fprintf(stderr, "back\t%d -> %d\n", a, b);
      first_cycle = false;
      while (!higher.empty() && d[higher.front()].first < d[b].first) {
	higher.pop_front();
      }
      lower.clear();
      /*fprintf(stderr, "higher:");
      for (int b : higher) {
	fprintf(stderr, " %d", b);
      }
      fprintf(stderr, "\n");*/
    } else {
      // forward or cross edge
      //fprintf(stderr, "forw\t%d -> %d\n", a, b);
      while (!lower.empty() && d[lower.front()].second > d[b].second) {
	lower.pop_front();
      }
    }
  }
  /*fprintf(stderr, "higher@%d:", a);
      for (int b : higher) {
	fprintf(stderr, " %d", b);
      }
      fprintf(stderr, "\n");*/
  if (!higher.empty() && higher.back() == a) {
    lower.push_front(a);
    higher.pop_back();
  }
  d[a].second = ++t;
}

bool rdfs_start(int a) {
  if (!d[a].first) {
    lower.clear();
    higher.clear();
    path_no = 0;
    first_cycle = true;
    scc[a] = ++scc_no;
    d[a].first = t = 1;
    higher.push_back(a);
    /*fprintf(stderr, "Radj[%d]:", a);
	for (int c : radj[a]) {
	  fprintf(stderr, " %d", c);
	}
	fprintf(stderr, "\n");*/
    for (int b : radj[a]) {
      if (!d[b].first) {
	++path_no;
	//fprintf(stderr, "Starting a path %d @ %d\n", b, a);
	rdfs(b);
      } else if (scc[b] == scc_no) {
	while (!lower.empty() && d[lower.front()].second > d[b].second) {
	  lower.pop_front();
	}
      }
    }
    if (!higher.empty() && higher.back() == a) {
      lower.push_front(a);
      higher.pop_back();
    }
    if (path_no) {
      needed.swap(lower);
    }
    //fprintf(stderr, "Got %d paths @ %d\n", path_no, a);
    return path_no;
  }
  return false;
}

int main() {
  scanf("%d%d", &n, &m);
  for (int i = 0; i < m; ++i) {
    int a, b;
    scanf("%d%d", &a, &b);
    adj[a].push_back(b);
    radj[b].push_back(a);
  }
  for (int i = 1; i <= n; ++i) {
    if (!v[i]) {
      dfs(i);
    }
  }
  bool cycle = false;
  while (!s.empty()) {
    int a = s.back();
    //fprintf(stderr, "Trying to start from %d\n", a);
    if (rdfs_start(a)) {
      if (cycle) {
	printf("0\n\n");
	return 0;
      } else {
	cycle = true;
      }
    }
    s.pop_back();
  }
  if (!cycle) {
    printf("NIE\n");
    return 0;
  }
  printf("%d\n", (int)needed.size());
  for (int a : needed) {
    is_needed[a] = true;
  }
  for (int i = 1; i <= n; ++i) {
    if (is_needed[i]) {
      printf("%d", i);
      for (++i; i <= n; ++i) {
	if (is_needed[i]) {
	  printf(" %d", i);
	}
      }
    }
  }
  printf("\n");
}