using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
using System.Threading;
using System.Runtime.CompilerServices;
static class Program{
	const int mod=(int)1e9+7;
	static void Main(){
		Sc sc=new Sc();
		var s=sc.Ia;
		int n=s[0],m=s[1];
		var li=new List<ValueTuple<int,int,int>>[n+1];
		var a=new int[n+1][];
		var g=new int[n+1][];
		for(int i=1;i<=n;i++){
			li[i]=new List<ValueTuple<int,int,int>>();
			a[i]=new int[]{0,0,0};
			g[i]=new int[]{0,0,0};
		}
		var h=new int[26];
		var ca=new char[m+1];
		for(int i=0;i<m;i++){
			var (e1,e2,e3)=sc.Tp3<int,char>();
			ca[i+1]=e3;
			li[e1].Add(ValueTuple.Create(e2,e3-'a',i+1));
			li[e2].Add(ValueTuple.Create(e1,e3-'a',i+1));
			h[e3-'a']++;
		}
		for(int i = 0;i<h.Length;i++) {
			if(h[i]==m){
				Console.WriteLine(-1);
				return;
			}
		}
		
		var qu=new Queue<ValueTuple<int,int,long>>();
		qu.Enqueue(ValueTuple.Create(1,0,0));
		while(qu.Count>0){
			var e=qu.Dequeue();
			var v=Min(2,Pc(e.Item1));
			for(int j = 0;j<li[e.Item1].Count;j++) {
				var p=e.Item3|(1L<<li[e.Item1][j].Item2);
				var d=Min(2,Pc(p));
				if(g[li[e.Item1][j].Item1][d]==0){
					qu.Enqueue((li[e.Item1][j].Item1,e.Item2+1,p));
					g[li[e.Item1][j].Item1][d]=e.Item1*(v!=d?-1:1);
					a[li[e.Item1][j].Item1][d]=li[e.Item1][j].Item3;
				}
			}
		}
		int z=n,f=2;
		if(g[z][f]==0){
			Console.WriteLine(-1);
			return;
		}
		var lia=new List<int>();
		while(f!=0){
			lia.Add(a[z][f]);
			if(g[z][f]<0){
				z=Abs(g[z][f]);
				f--;
			}
			else{z=Abs(g[z][f]);}
		}
		lia.Reverse();
		var c=new int[lia.Count];
		for(int i = 0;i<c.Length;i++) {c[i]=ca[lia[i]];}
		var bo=false;
		for(int i = 0;i<c.Length;i++) {
			if(c[i]!=c[c.Length-i-1]){
				bo=true;
				break;
			}
		}
		if(!bo){
			lia.Add(lia[lia.Count-1]);
			lia.Add(lia[lia.Count-1]);
		}
		Console.WriteLine("{0}",lia.Count);
		Console.WriteLine("{0}",string.Join("\n",lia));
	}
	static int Pc(long bits){
		bits=(bits&0x55555555)+(bits>>1&0x55555555);
		bits=(bits&0x33333333)+(bits>>2&0x33333333);
		bits=(bits&0x0f0f0f0f)+(bits>>4&0x0f0f0f0f);
		bits=(bits&0x00ff00ff)+(bits>>8&0x00ff00ff);
		return (int)((bits&0x0000ffff)+(bits>>16&0x0000ffff));
	}
}
public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(string a,string b){return Array.ConvertAll((a+Console.ReadLine()+b).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(string a,string b){return Array.ConvertAll((a+Console.ReadLine()+b).Split(),long.Parse);}
	public double[] Da2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),double.Parse);}}
	public double[] Da3(string a,string b){return Array.ConvertAll((a+Console.ReadLine()+b).Split(),double.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
	public (T,T) Tp2<T>(){var s=Console.ReadLine().Split();return (Ct<T>(s[0]),Ct<T>(s[1]));}
	public (T1,T2) Tp2<T1,T2>(){var s=Console.ReadLine().Split();return (Ct<T1>(s[0]),Ct<T2>(s[1]));}
	public (T,T,T) Tp3<T>(){var s=Console.ReadLine().Split();return (Ct<T>(s[0]),Ct<T>(s[1]),Ct<T>(s[2]));}
	public (T1,T1,T2) Tp3<T1,T2>(){var s=Console.ReadLine().Split();return (Ct<T1>(s[0]),Ct<T1>(s[1]),Ct<T2>(s[2]));}
	private T Ct<T>(string s){return (T)Convert.ChangeType(s,typeof(T));}
}