#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;
using ll = long long;

#include<set>
int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int n,m;
    cin>>n>>m;
    vector<vector<pair<int,pair<char,int>>>> g(n);
    vector<char> C(m);
    set<char> s;
    for(int i = 0;i<m;i++){
        int u,v;
        char c;
        cin>>u>>v>>c;
        C[i] = c;
        u--;v--;
        s.insert(c);
        g[u].push_back({v,{c,i+1}});
        g[v].push_back({u,{c,i+1}});
    }
    if(s.size()==1){
        cout<<-1<<endl;
        return 0;
    }
    if(n==2){
        int ni = 0,nj;
        for(int i = 0;i<g[0].size();i++){
            if(g[0][0].second.first==g[0][i].second.first) continue;
            nj = i;
            break;
        }
        cout<<3<<endl;
        cout<<ni+1<<endl<<nj+1<<endl<<nj+1<<endl;
        return 0;
    }else if(n==3){
        vector<int> ans;
        for(int i = 0;i<3;i++){
            set<int> a;
            set<char> b;
            vector<int> use;
            for(int j = 0;j<g[i].size();j++){
                if(b.find(g[i][j].second.first)!=b.end()) continue;
                if(a.find(g[i][j].first)!=a.end()) continue;
                b.insert(g[i][j].second.first);
                a.insert(g[i][j].first);
                use.push_back(j);
            }
            if(a.size()<2) continue;
            int ni = use[0];
            int nj = use[1];
            if(i==0){
                if(g[0][ni].first==2) swap(ni,nj);
                ans.push_back(g[0][ni].second.second);
                ans.push_back(g[0][ni].second.second);
                ans.push_back(g[0][nj].second.second);
                break;
            }else if(i==1){
               if(g[i][ni].first==2) swap(ni,nj);
                ans.push_back(g[0][ni].second.second);
                ans.push_back(g[0][nj].second.second);
                break;
            }else{
               if(g[i][ni].first==1) swap(ni,nj);
                ans.push_back(g[0][ni].second.second);
                ans.push_back(g[0][nj].second.second);
                ans.push_back(g[0][nj].second.second);
                break;
            }
        }
        cout<<ans.size()<<endl;
        for(int i = 0;i<ans.size();i++) cout<<ans[i]<<endl;
        return 0;
    }
    char no = g[n-1][0].second.first;
    int wu,ni;
    ni = -1;
    int wv;
    for(int i = 0;i<n;i++){
        set<char> now;
        for(auto itr:g[i]){
            now.insert(itr.second.first);
        }
        if(now.size()==1) continue;
        wu = i;
        break;
    }
    vector<int> vis(n,0);
    bool fn = false;
    vector<int> que;
    auto dfs = [&](auto dfs,int ni,int to) -> void {
        vis[ni]++;
        if(ni==to){
            fn = true;
            return;
        }
        for(auto&itr:g[ni]){
            int nj = itr.first;
            if(vis[nj]) continue;
            que.push_back(itr.second.second);
            dfs(dfs,nj,to);
            if(fn) return;
            que.pop_back();
        }
    };
    dfs(dfs,0,wu);
    vector<int> ans = que;
    que.clear();
    int k = wu;
    if(wu==0){
        int ni = 0;
        int nj ;
        for(int j = 1;j<g[0].size();j++){
            if(g[0][j].second.first!=g[0][0].second.first){
                nj = j;
                break;
            }
        }
        ans.push_back(ni);
        ans.push_back(ni);
        ans.push_back(nj);
        k = g[0][nj].second.second;
    }
    for(int i = 0;i<n;i++) vis[i] = 0;
    int from = k;
    ans.push_back(ni);
    int to = n - 1;
    fn = false;
    dfs(dfs,from,to);
    for(int i:que) ans.push_back(i);

    string use;
    for(int i = 0;i<ans.size();i++){
        use += C[ans[i]-1];
    }
    string u = use;
    reverse(u.begin(),u.end());
    if(use==u){
        ans.push_back(g[n-1][0].second.second);
        ans.push_back(g[n-1][0].second.second);
    }
    cout<<ans.size()<<endl;
    for(int i = 0;i<ans.size();i++) cout<<ans[i]<<endl;
}