#include <cstdio>
#include <vector>
#include <algorithm>

const int MAX_N = 1000;

int n;
std::vector<int> g[1 + 2 * MAX_N];
std::vector<int> trans[1 + 2 * MAX_N];
std::vector<int> topo;
std::vector<int> sol;
bool viz[1 + 2 * MAX_N];
bool val[1 + 2 * MAX_N];
bool possible = true;

int conj(int x) {
  if (x <= n)
    return x + n;
  return x - n;
}

void make_edge(int u, int v) {
  g[u].push_back(v);
  trans[v].push_back(u);
}

void topoSort(int u) {
  viz[u] = true;
  for (auto v : g[u]) {
    if (!viz[v])
      topoSort(v);
  }
  topo.push_back(u);
}

void markNodes(int u) {
  viz[u] = true;
  if (val[u]) {
    possible = false;
    return;
  }
  val[u] = false;
  val[conj(u)] = true;
  for (auto v : trans[u]) {
    if (!viz[v])
      markNodes(v);
  }
}

int main() {
  freopen("party.in", "r", stdin);
  freopen("party.out", "w", stdout);

  int m;
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) {
    int x, y, restriction;
    scanf("%d%d%d", &x, &y, &restriction);
    switch (restriction) {
      case 0:
        make_edge(conj(x), y);
        make_edge(conj(y), x);
        break;
      case 1:
        make_edge(conj(x), conj(y));
        break;
      case 2:
        make_edge(conj(y), conj(x));
        break;
      case 3:
        make_edge(x, conj(y));
        make_edge(y, conj(x));
        break;
    }
  }
  for (int i = 1; i <= 2 * n; i++) {
    if (!viz[i])
      topoSort(i);
  }
  std::reverse(topo.begin(), topo.end());
  for (int i = 1; i <= 2 * n; i++)
    viz[i] = false;
  for (auto node : topo) {
    if (!viz[node] && !viz[conj(node)])
      markNodes(node);
    if (!possible) {
      printf("-1\n");
      return 0;
    }
  }
  for (int i = 1; i <= n; i++) {
    if (val[i])
      sol.push_back(i);
  }
  printf("%d\n", sol.size());
  for (auto node : sol)
    printf("%d\n", node);
  printf("\n");
  return 0;
}