#include <bits/stdc++.h>
using ll = long long;
using std::cin;
using std::cout;
using std::endl;
std::mt19937 rnd(std::chrono::steady_clock::now().time_since_epoch().count());
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const int inf = (int)1e9 + 7;
const long long INF = 1LL << 60;


bool palindrome(const std::string &s)
{
  int l = 0;
  int r = (int)s.size() - 1;
  while (l < r)
  {
    if(s[l] != s[r])
      return false;
    l += 1, r -= 1;
  }
  return true;
}
void solve()
{
  int n, m; cin >> n >> m;
  std::vector<std::vector<std::pair<int, char>>> g(n);
  std::vector<std::vector<int>> edge(26);
  std::vector<std::pair<int,int>> vp(m);
  std::vector<char> c(m);
  for (int i = 0; i < m; ++i)
  {
    int u, v; cin >> u >> v;
    u -= 1, v -= 1;
    vp[i] = {u, v};
    cin >> c[i];
    g[u].emplace_back(v, i);
    g[v].emplace_back(u, i);
    edge[c[i] - 'a'].emplace_back(i);
  }
  std::vector<std::pair<int, int>> e(2, { -1, -1 });
  std::vector<int> eid(2, -1);
  for (int i = 0; i < 26; ++i)
  {
    if(edge[i].empty())
      continue;
    if(e[0].first == -1)
    {
      eid[0] = edge[i][0];
      e[0] = vp[edge[i][0]];
    }
    else
    {
      eid[1] = edge[i][0];
      e[1] = vp[edge[i][0]];
    }
  }
  if(e[1].first == -1)
  {
    cout << -1 << "\n";
    return;
  }
  {
    std::vector<int> d(n, inf);
    std::queue<int> q;
    q.emplace(0);
    d[0] = 0;
    while(not q.empty())
    {
      const int cur = q.front();
      q.pop();
      for (const auto &[nxt, id] : g[cur])
      {
        if(chmin(d[nxt], d[cur] + 1))
        {
          q.emplace(nxt);
        }
      }
    }
    for (int i = 0; i < 2; i++)
      if(d[e[i].first] > d[e[i].second])
        std::swap(e[i].first, e[i].second);
    if(d[e[0].first] > d[e[1].first])
    {
      std::swap(eid[0], eid[1]);
      std::swap(e[0], e[1]);
    }
  }
  std::vector<int> res;
  std::string s;
  {
    const int st = 0;
    std::vector<int> d(n, inf), b(n, -1), bid(n, -1);
    std::queue<int> q;
    q.emplace(st);
    d[st] = 0;
    while(not q.empty())
    {
      const int cur = q.front();
      q.pop();
      for (const auto &[nxt, id] : g[cur])
      {
        if(chmin(d[nxt], d[cur] + 1))
        {
          b[nxt] = cur;
          bid[nxt] = id;
          q.emplace(nxt);
        }
      }
    }
    std::string t;
    std::vector<int> ans;
    int to = e[0].first;
    while (to != st)
    {
      t += c[bid[to]];
      ans.emplace_back(bid[to]);
      to = b[to];
    }
    std::reverse(t.begin(), t.end());
    std::reverse(ans.begin(), ans.end());
    s += t;
    for (const auto &e : ans)
      res.emplace_back(e);
  }
  s += c[eid[0]];
  res.emplace_back(eid[0]);
  {
    const int st = e[0].second;
    std::vector<int> d(n, inf), b(n, -1), bid(n, -1);
    std::queue<int> q;
    q.emplace(st);
    d[st] = 0;
    while(not q.empty())
    {
      const int cur = q.front();
      q.pop();
      for (const auto &[nxt, id] : g[cur])
      {
        if(chmin(d[nxt], d[cur] + 1))
        {
          b[nxt] = cur;
          bid[nxt] = id;
          q.emplace(nxt);
        }
      }
    }
    if(d[e[1].first] > d[e[1].second])
      std::swap(e[1].first, e[1].second);
    std::string t;
    std::vector<int> ans;
    int to = e[1].first;
    while (to != st)
    {
      t += c[bid[to]];
      ans.emplace_back(bid[to]);
      to = b[to];
    }
    std::reverse(t.begin(), t.end());
    std::reverse(ans.begin(), ans.end());
    s += t;
    for (const auto &e : ans)
      res.emplace_back(e);
  }
  s += c[eid[1]];
  res.emplace_back(eid[1]);
  {
    const int st = e[1].second;
    std::vector<int> d(n, inf), b(n, -1), bid(n, -1);
    std::queue<int> q;
    q.emplace(st);
    d[st] = 0;
    while(not q.empty())
    {
      const int cur = q.front();
      q.pop();
      for (const auto &[nxt, id] : g[cur])
      {
        if(chmin(d[nxt], d[cur] + 1))
        {
          b[nxt] = cur;
          bid[nxt] = id;
          q.emplace(nxt);
        }
      }
    }
    std::string t;
    std::vector<int> ans;
    int to = n - 1;
    while (to != st)
    {
      t += c[bid[to]];
      ans.emplace_back(bid[to]);
      to = b[to];
    }
    std::reverse(t.begin(), t.end());
    std::reverse(ans.begin(), ans.end());
    s += t;
    for (const auto &e : ans)
      res.emplace_back(e);
  }
  if (palindrome(s))
  {
    s += c[g[n - 1][0].second];
    s += c[g[n - 1][0].second];
    res.emplace_back(g[n - 1][0].second);
    res.emplace_back(g[n - 1][0].second);
  }
  if(palindrome(s))
  {
    assert(false);
  }
  cout << res.size() << "\n";
  for (const auto & e : res)
    cout << e + 1 << "\n";
}
int main()
{
  std::cin.tie(nullptr);
  std::ios::sync_with_stdio(false);

  int kkt = 1; 
  // cin >> kkt;
  while(kkt--)
    solve();
  return 0;
  
}