#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
constexpr int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

template <typename CostType>
struct Edge {
  int src, dst; CostType cost;
  Edge(int src, int dst, CostType cost = 0) : src(src), dst(dst), cost(cost) {}
  inline bool operator<(const Edge &x) const {
    return cost != x.cost ? cost < x.cost : dst != x.dst ? dst < x.dst : src < x.src;
  }
  inline bool operator<=(const Edge &x) const { return !(x < *this); }
  inline bool operator>(const Edge &x) const { return x < *this; }
  inline bool operator>=(const Edge &x) const { return !(*this < x); }
};

int main() {
  int n, m; cin >> n >> m;
  vector<int> a(m), b(m);
  vector<char> c(m);
  vector<vector<int>> graph(n);
  REP(i, m) {
    cin >> a[i] >> b[i] >> c[i]; --a[i]; --b[i];
    graph[a[i]].emplace_back(i);
    graph[b[i]].emplace_back(i);
  }
  if (set<char>(ALL(c)).size() == 1) {
    cout << "-1\n";
    return 0;
  }
  vector<int> prev(n, -1);
  prev[0] = -2;
  queue<int> que({0});
  while (!que.empty()) {
    int ver = que.front(); que.pop();
    for (int id : graph[ver]) {
      const int dst = (a[id] == ver ? b : a)[id];
      if (prev[dst] == -1) {
        prev[dst] = id;
        que.emplace(dst);
      }
    }
  }
  vector<int> ans;
  string s = "";
  int cur = n - 1;
  while (cur != 0) {
    ans.emplace_back(prev[cur]);
    s += c[prev[cur]];
    cur = (a[prev[cur]] == cur ? b : a)[prev[cur]];
  }
  reverse(ALL(ans));
  reverse(ALL(s));
  if (equal(ALL(s), s.rbegin())) {
    if (set<char>(ALL(s)).size() > 1) {
      ans.resize(n * 2, ans.back());
    } else {
      const char ch = s.front();
      fill(ALL(prev), -1);
      for (int id : ans) prev[a[id]] = prev[b[id]] = -2;
      REP(i, n) que.emplace(i);
      cur = -1;
      while (!que.empty()) {
        int ver = que.front(); que.pop();
        for (int id : graph[ver]) {
          const int dst = (a[id] == ver ? b : a)[id];
          if (prev[dst] == -1) {
            prev[dst] = id;
            que.emplace(dst);
            if (cur == -1 && c[id] != ch) cur = dst;
          }
        }
      }
      assert(cur != -1);
      vector<int> sub_ans;
      string sub_s = "";
      while (prev[cur] != -2) {
        sub_ans.emplace_back(prev[cur]);
        sub_s += c[prev[cur]];
        cur = (a[prev[cur]] == cur ? b : a)[prev[cur]];
      }
      reverse(ALL(sub_ans));
      reverse(ALL(sub_s));
      for (int i = static_cast<int>(sub_ans.size()) - 1; i >= 0; --i) {
        sub_ans.emplace_back(sub_ans[i]);
        sub_s += sub_s[i];
      }
      if (cur == 0) {
        ans.insert(ans.begin(), ALL(sub_ans));
        s.insert(s.begin(), ALL(sub_s));
      } else {
        int pos = 0;
        REP(i, ans.size()) {
          pos = (a[ans[i]] == pos ? b : a)[ans[i]];
          if (pos == cur) {
            ans.insert(ans.begin() + (i + 1), ALL(sub_ans));
            s.insert(s.begin() + (i + 1), ALL(sub_s));
            break;
          }
        }
      }
      if (equal(ALL(s), s.rbegin())) ans.resize(n * 2, ans.back());
    }
  }
  const int k = ans.size();
  cout << k << '\n';
  REP(i, k) cout << ans[i] + 1 << '\n';
  return 0;
}