#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define Pr pair<ll,ll>
#define Tp tuple<ll,ll,ll>
using Graph = vector<vector<tuple<int,int,char>>>;

const ll mod = 1000000007;

int main() {
    ll N,M;
    cin >> N >> M;
    Graph G(N+1);
    vector<tuple<ll,ll,char>> path;
    vector<set<char>> cnt(N+1);
    rep(i,M){
        ll a,b; char c; cin >> a >> b >> c;
        G[a].push_back(make_tuple(b,i+1,c));
        G[b].push_back(make_tuple(a,i+1,c));
        path.push_back(make_tuple(a,b,c));
        cnt[a].insert(c); cnt[b].insert(c);
    }
    ll pt = 0;
    rep(i,N){
        if(cnt[i+1].size()>1){
            pt = i+1; break;
        }
    }
    if(pt==0){
        cout << -1 << endl; return 0;
    }
    char d1,d2; int a1,a2;
    auto[aa,kk,dd] = G[pt][0]; a1 = kk; d1 = dd;
    int ss = aa; int tt;
    rep(i,G[pt].size()){
        auto[a,k,d] = G[pt][i];
        if(d!=d1){
            a2 = k; tt = a; d2 = d; break;
        }
    }
    //cout << a1 << " " << a2 << endl;
    //cout << ss  << " " << tt << endl;
    string S; vector<ll> ans;
    //BFS (普通の幅優先探索)
    queue<int> go; ll dist[N+1],prev[N+1],used[N+1]; // ll par[N+1];
    char cc[N+1];
    rep(i,N+1){
        dist[i] = 2000000000; prev[i] = 0; used[i] = 0;
    }
    dist[tt] = 0; go.push(tt);
    while(!go.empty()){
        int s = go.front(); go.pop();
        for(auto p:G[s]){
            auto[x,k,d] = p;
            if(dist[x]<=dist[s]+1) continue;
            dist[x] = dist[s] + 1;
            prev[x] = s; used[x] = k; cc[x] = d;
            go.push(x);
        }
    }
    int n = N;
    while(prev[n]>0){
        ans.push_back(used[n]); S += cc[n];
        n = prev[n];
    }

    S += d2; S += d1;
    ans.push_back(a2); ans.push_back(a1);

    rep(i,N+1){
        dist[i] = 2000000000; prev[i] = 0; used[i] = 0;
    }
    dist[1] = 0; go.push(1);
    while(!go.empty()){
        int s = go.front(); go.pop();
        for(auto p:G[s]){
            auto[x,k,d] = p;
            if(dist[x]<=dist[s]+1) continue;
            dist[x] = dist[s] + 1;
            prev[x] = s; used[x] = k; cc[x] = d;
            go.push(x);
        }
    }
    n = ss;
    while(prev[n]>0){
        ans.push_back(used[n]); S += cc[n];
        n = prev[n];
    }
    
    string T = S;
    reverse(S.begin(),S.end());
    if(S==T){
        auto[a,k,d] = G[1][0];
        ans.push_back(k); ans.push_back(k);
    }
    reverse(ans.begin(),ans.end());
    cout << ans.size() << endl;
    for(ll yy:ans){
        cout << yy << endl;
    }
    //cout << ans << endl;
}