#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using P = pair<int, int>;
using PL = pair<lint, lint>;
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
#define ALL(a)  (a).begin(),(a).end()
constexpr int MOD = 1000000007;
constexpr lint B1 = 1532834020;
constexpr lint M1 = 2147482409;
constexpr lint B2 = 1388622299;
constexpr lint M2 = 2147478017;
constexpr int INF = 2147483647;
void yes(bool expr) {cout << (expr ? "Yes" : "No") << "\n";}
template<class T>void chmax(T &a, const T &b) { if (a<b) a=b; }
template<class T>void chmin(T &a, const T &b) { if (b<a) a=b; }
struct Node;
vector<Node> V;
int N, M;
struct Node {
  int idx;
  vector<tuple<int, char, int>> nbr;
  set<int> vst;
  void dfs(vector<int> &e, int mode) {
    if(vst.find(mode) != vst.end()) return;
    vst.insert(mode);
    if(mode == 26 && idx == N-1) {
      if(e.size() > 2 * N) return;
      while(e.size() <= 2 * N - 2) {
        e.push_back(e[e.size()-1]);
        e.push_back(e[e.size()-1]);
      }
      cout << e.size() << "\n";
      REP(i, e.size()) cout << e[i] << "\n";
      exit(0);
    }
    for(auto t : nbr) {
      int n = get<0>(t);
      int c = get<1>(t) - 'a';
      int ei = get<2>(t);
      int nmode = c;
      if(mode == 26 || (mode != -1 && mode != c)) nmode = 26;
      e.push_back(ei);
      V[n].dfs(e, nmode);
      e.pop_back();
    }
  };
};
int main()
{
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> N >> M;
  V = vector<Node>(N);
  REP(i, N) V[i].idx = i;
  set<char> st;
  REP(i, M) {
    int A, B;
    char C;
    cin >> A >> B >> C;
    st.insert(C);
    A--; B--;
    V[A].nbr.push_back(make_tuple(B, C, i+1));
    V[B].nbr.push_back(make_tuple(A, C, i+1));
  }
  if(st.size() == 1) {
    cout << -1 << endl;
    return 0;
  }
  vector<int> e;
  V[0].dfs(e, -1);
}