#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define each(e, v) for(auto &e: v)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const int MOD = 1000000007;
//const int MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

struct io_setup{
    io_setup(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
    }
} io_setup;

struct Union_Find_Tree{
    vector<int> data;
    const int n;
    
    Union_Find_Tree(int n) : data(n, -1), n(n) {}
    
    int root(int x){
        if(data[x] < 0) return x;
        return data[x] = root(data[x]);
    }

    int operator [] (int i) {return root(i);}
    
    bool unite(int x, int y){
        x = root(x), y = root(y);
        if(x == y) return false;
        if(data[x] > data[y]) swap(x, y);
        data[x] += data[y], data[y] = x;
        return true;
    }
    
    int size(int x) {return -data[root(x)];}
    
    bool same(int x, int y) {return root(x) == root(y);}
    
    void clear() {fill(begin(data), end(data), -1);}
};

template<typename T, bool directed = false>
struct Weighted_Graph{
    struct edge{
        int to; T cost; int id;
        edge(int to, T cost, int id) : to(to), cost(cost), id(id) {}
    };

    vector<vector<edge>> es;
    const T INF_T;
    const int n;
    int m;

    vector<int> used;

    Weighted_Graph(int n) : es(n), INF_T(numeric_limits<T>::max()/2), n(n), m(0), used(n) {}

    void add_edge(int from, int to, T cost){
        es[from].emplace_back(to, cost, m);
        if(!directed) es[to].emplace_back(from, cost, m);
        m++;
    }

    vector<int> ret_path;

    bool detect_path(int now, int t, bool use_id = false){
        used[now] = true;
        if(now == t){
            if(!use_id) ret_path.push_back(now);
            return true;
        }
        for(auto &e : es[now]){
            if(used[e.to]) continue;
            if(detect_path(e.to, t, use_id)){
                ret_path.push_back(use_id? e.id : now);
                return true;
            }
        }
        return false;
    }

    vector<int> find_path(int s, int t, bool use_id = false){ //点素なs-tパスを1つ検出
        ret_path.clear(), fill(begin(used), end(used), 0);
        detect_path(s, t, use_id);
        reverse(begin(ret_path), end(ret_path));
        return ret_path;
    }
};

int main(){
    int N, M; cin >> N >> M;

    vector<int> u(M), v(M);
    vector<char> c(M);

    Weighted_Graph<char> G(N);

    rep(i, M){
        cin >> u[i] >> v[i] >> c[i]; u[i]--, v[i]--;
        G.add_edge(u[i], v[i], c[i]);
    }

    Union_Find_Tree uf(N);

    vector<int> ok;

    rep(i, 26){
        rep(j, M){
            if(c[j]-'a' == i){
                uf.unite(u[j], v[j]);
            }
        }
        if(uf.same(0, N-1)) ok.eb(i);
        uf.clear();
    }

    if(sz(ok) != 1){
        vector<int> path = G.find_path(0, N-1, true);
        if(sz(ok) != 0){
            each(e, G.es[N-1]){
                if(e.cost != c[path.back()]){
                    path.eb(e.id), path.eb(e.id);
                    break;
                }
            }
        }
        else{
            vector<char> s;
            each(e, path) s.eb(c[e]);
            int n = sz(s);
            bool flag = true;
            rep(i, n){
                if(s[i] != s[n-1-i]) flag = false;
            }

            if(flag){
                path.eb(path.back()), path.eb(path.back());
            }
        }

        int n = sz(path);
        cout << n << '\n';
        rep(i, n) cout << path[i]+1 << '\n';
    }
    else{
        int id = -1;
        rep(i, M){
            if(c[i]-'a' != ok[0]) id = i;
        }

        //cout << ok[0] << ' ' << id << '\n';

        if(id == -1){
            cout << "-1\n";
            return 0;
        }

        vector<int> p1 = G.find_path(0, u[id], true), p2 = G.find_path(v[id], N-1, true);

        if(sz(p1) == N-1 && sz(p2) == N-1){
            p1 = G.find_path(0, v[id], true), p2 = G.find_path(u[id], N-1, true);
        }
        
        vector<int> path;
        each(e, p1) path.eb(e);
        path.eb(id);
        each(e, p2) path.eb(e);

        if(sz(p1) == sz(p2)){
            rep(i, 2){
                path.eb(path.back());
            }
        }

        cout << sz(path) << '\n';
        each(e, path) cout << e+1 << '\n';
    }
}