using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    static string[] SList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => ReadLine()).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (n, m) = (c[0], c[1]);
        var map = NArr(m);
        var ans = Route(n, m, map);
        WriteLine(ans);
    }
    static string Route(int n, int m, int[][] map)
    {
        var tree = new List<(int id, int to, long len)>[n];
        for (var i = 0; i < n; ++i) tree[i] = new List<(int id, int to, long len)>();
        for (var i = 0; i < m; ++i)
        {
            var edge = map[i];
            tree[edge[0] - 1].Add((i, edge[1] - 1, edge[2]));
            tree[edge[1] - 1].Add((i, edge[0] - 1, -edge[2]));
        }
        var len = Enumerable.Repeat(INF, n).ToArray();
        var ans = "";
        for (var i = 0; i < n; ++i)
        {
            if (len[i] == INF)
            {
                len[i] = 0;
                if (DFS(i, tree, len, new List<int>(), new List<int>(), ref ans)) return ans;
            }
        }
        return "-1";
    }
    static long INF = long.MinValue;
    static bool DFS(int cur, List<(int id, int to, long len)>[] tree, long[] len, List<int> vlist, List<int> elist, ref string ans)
    {
        foreach (var next in tree[cur])
        {
            if (len[next.to] == INF)
            {
                len[next.to] = len[cur] + next.len;
                vlist.Add(cur);
                elist.Add(next.id);
                if (DFS(next.to, tree, len, vlist, elist, ref ans))
                {
                    return true;
                }
                vlist.RemoveAt(vlist.Count - 1);
                elist.RemoveAt(elist.Count - 1);
            }
            else
            {
                if (len[next.to] == len[cur] + next.len) continue;
                var start = vlist.LastIndexOf(next.to);
                var route = elist.Skip(start).ToList();
                route.Insert(0, next.id);
                if (len[next.to] > len[cur] + next.len) route.Reverse();
                ans = $"{route.Count}\n{cur + 1}\n{string.Join(" ", route.Select(r => r + 1))}";
                return true;
            }
        }
        return false;
    }
}