#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000

bool check(string s){
	string t(s.rbegin(),s.rend());
	return s!=t;
}

int main(){
	
	int N,M;
	cin>>N>>M;
	
	vector<int> A(M),B(M);
	vector<int> C(M);
	
	vector<vector<int>> E(N);
	
	rep(i,M){
		char c;
		cin>>A[i]>>B[i]>>c;
		A[i]--;B[i]--;
		C[i] = c - 'a';
		E[A[i]].push_back(i);
		E[B[i]].push_back(i);
	}
	
	array<int,3> AA = {Inf,Inf,Inf};
	
	vector pre(N,vector(27,vector<array<int,3>>(2,AA)));
	vector eind(N,vector(27,vector<int>(2,-1)));
	
	pre[0][26][0] = {-1,-1,-1};
	queue<array<int,3>> Q;
	Q.push({0,26,0});
	
	while(Q.size()>0){
		int u = Q.front()[0],c = Q.front()[1],ff = Q.front()[2];
		Q.pop();
		
		rep(i,E[u].size()){
			int ind = E[u][i];
			int v = u ^ A[ind] ^ B[ind];
			char nc = C[ind];
			int nf = ff;
			if(c!=26 && nc!=c)nf = 1;
			if(pre[v][nc][nf]!=AA)continue;
			pre[v][nc][nf] = {u,c,ff};
			eind[v][nc][nf] = ind;
			Q.push({v,nc,nf});
		}
	}
	
	rep(i,26){
		if(pre.back()[i][1]!=AA){
			string S = "";
			array<int,3> cur = {N-1,i,1};
			vector<int> ans;
			while(cur[0]!=-1){
				S += 'a' + cur[1];
				ans.push_back(eind[cur[0]][cur[1]][cur[2]]);
				cur = pre[cur[0]][cur[1]][cur[2]];
			}
			S.pop_back();
			ans.pop_back();
			reverse(ans.begin(),ans.end());
			reverse(S.begin(),S.end());
			while(!check(S)){
				S += 'a'+i;
				S += 'a'+i;
				ans.push_back(eind[N-1][i][1]);
				ans.push_back(eind[N-1][i][1]);
			}
			cout<<ans.size()<<endl;
			rep(i,ans.size()){
				cout<<ans[i]+1<<endl;
			}
			return 0;
		}
	}
	
	cout<<-1<<endl;
	
	
	return 0;
}