#include <bits/stdc++.h>
using namespace std;
vector<int> z_algorithm(string S){
	int N = S.size();
	vector<int> ans(N);
	for (int i = 1, j = 0; i < N; i++){
		if (i + ans[i - j] < j + ans[j]){
			ans[i] = ans[i - j];
		} else {
			int k = max(0, j + ans[j] - i);
			while (i + k < N && S[k] == S[i + k]){
				k++;
			}
			ans[i] = k;
			j = i;
		}
	}
	ans[0] = N;
	return ans;
}
int main(){
  int N, M;
  cin >> N >> M;
  vector<vector<tuple<char, int, int>>> E(N);
  for (int i = 0; i < M; i++){
    int A, B;
    char C;
    cin >> A >> B >> C;
    A--;
    B--;
    E[A].push_back(make_tuple(C, i, B));
    E[B].push_back(make_tuple(C, i, A));
  }
  vector<vector<int>> d(27, vector<int>(N, -1));
  vector<vector<int>> pv(27, vector<int>(N, -1));
  vector<vector<int>> pe(27, vector<int>(N, -1));
  vector<vector<int>> ps(27, vector<int>(N, -1));
  vector<vector<char>> pc(27, vector<char>(N, -1));
  queue<pair<int, int>> Q;
  for (int i = 0; i < 26; i++){
    d[i][0] = 0;
    Q.push(make_pair(i, 0));
  }
  while (!Q.empty()){
    int s = Q.front().first;
    int v = Q.front().second;
    Q.pop();
    for (auto e : E[v]){
      char c = get<0>(e);
      if (!(v == 0 && s != c - 'a')){
        int w = get<2>(e);
        int s2 = s;
        if (s != c - 'a'){
          s2 = 26;
        }
        if (d[s2][w] == -1){
          d[s2][w] = d[s][v] + 1;
          pv[s2][w] = v;
          pe[s2][w] = get<1>(e);
          ps[s2][w] = s;
          pc[s2][w] = c;
          Q.push(make_pair(s2, w));
        }
      }
    }
  }
  if (d[26][N - 1] == -1){
    cout << -1 << endl;
  } else {
    string S;
    vector<int> ans;
    int c = N - 1;
    int s = 26;
    while (c != 0 || s == 26){
      S += pc[s][c];
      ans.push_back(pe[s][c]);
      int c2 = pv[s][c];
      int s2 = ps[s][c];
      c = c2;
      s = s2;
    }
    reverse(S.begin(), S.end());
    reverse(ans.begin(), ans.end());
    int cnt = 0;
    while (S.size() + 2 <= N * 2){
      char ch = pc[26][N - 1];
      S += ch;
      S += ch;
      ans.push_back(pe[26][N - 1]);
      ans.push_back(pe[26][N - 1]);
      cnt += 2;
    }
    string S2 = S;
    reverse(S2.begin(), S2.end());
    string T = S + '$' + S2;
    vector<int> Z = z_algorithm(T);
    int p = -1;
    for (int i = 0; i <= cnt; i += 2){
      if (Z[S.size() + 1 + i] != S.size() - i){
        p = i;
        break;
      }
    }
    if (p == -1){
      cout << -1 << endl;
    } else {
      for (int i = 0; i < cnt - p; i++){
        ans.pop_back();
      }
      int k = ans.size();
      cout << k << endl;
      for (int i = 0; i < k; i++){
        cout << ans[i] + 1 << endl;
      }
    }
  }
}