import std.stdio, std.array, std.string, std.conv, std.algorithm;
import std.typecons, std.range, std.random, std.math, std.container;
import std.numeric, std.bigint, core.bitop, std.datetime;


void main() {
    auto s = readln.split.map!(to!int);
    auto N = s[0];
    auto M = s[1];
    auto G = new Tuple!(int, int, int)[][](N);
    int[] Cs;
    foreach (i; 0..M) {
        auto t = readln.split;
        auto u = t[0].to!int - 1;
        auto v = t[1].to!int - 1;
        auto c = (t[2].to!char - 'a').to!int;
        G[u] ~= tuple(v, c, i);
        G[v] ~= tuple(u, c, i);
        Cs ~= c;
    }

    if (Cs.sort().uniq.array.length == 1) {
        writeln(-1);
        return;
    }

    int mid;

    foreach (i; 0..N) {
        if (G[i].map!(t => t[1]).array.sort().uniq.array.length >= 2) {
            mid = i;
            break;
        }
    }

    auto H = new Edge!int[][](N);
    foreach (i; 0..N) foreach (j; G[i]) H[i] ~= Edge!int(j[0], 1);

    auto path1 = dijkstra!(int, 1<<29)(H, 0, mid);
    auto path2 = dijkstra!(int, 1<<29)(H, mid, N-1);

    int[] S;
    int[] ans1;
    int[] ans2;
    foreach (i; 0..path1.length.to!int-1) foreach (j; G[path1[i]]) if (j[0] == path1[i+1]) {
        S ~= j[1];
        ans1 ~= j[2];
    }
    foreach (i; 0..path2.length.to!int-1) foreach (j; G[path2[i]]) if (j[0] == path2[i+1]) {
        S ~= j[1];
        ans2 ~= j[2];
    }


    if (S.dup.sort().uniq.array.length == 1) {
        foreach (j; G[mid]) if (j[1] != S.front) {
            ans1 ~= j[2];
            ans1 ~= j[2];
            break;
        }
    }
    int[] ans = ans1 ~ ans2;

    if (is_kaibun(S)) {
        if (path1.length > 1) {
            ans = [ans.front, ans.front] ~ ans;
        } else if (path2.length > 1) {
            ans = ans ~ [ans.back, ans.back];
        }
    }

    writeln(ans.length);
    ans.each!(a => (a+1).writeln);

}

bool is_kaibun(int[] S) {
    foreach (i; 0..S.length.to!int/2) {
        if (S[i] != S[S.length.to!int-i-1]) return false;
    }
    return true;
}

struct Edge(T) {
    int to;
    T cost;
}

int[] dijkstra(T, T inf)(const Edge!(T)[][] graph, int start, int goal) {
    import std.typecons : Tuple, tuple;
    import std.conv : to;
    import std.container : BinaryHeap;

    int n = graph.length.to!int;
    auto dist = new T[](n);
    dist[] = inf;
    dist[start] = 0;

    int[] prev = new int[](n);
    prev[] = -1;

    auto pq = new BinaryHeap!(Array!(Edge!(T)), "a.cost > b.cost");
    pq.insert(Edge!(T)(start, 0));

    while (!pq.empty) {
        auto u = pq.front.to;
        auto cost = pq.front.cost;
        pq.removeFront;
        foreach (const e; graph[u]) {
            auto v = e.to;
            auto next_cost = cost + e.cost;
            if (dist[v] <= next_cost) continue;
            dist[v] = next_cost;
            prev[v] = u;
            pq.insert(Edge!(T)(v, next_cost));
        }
    }

    int[] ans;
    for (int cur = goal; cur != start; cur = prev[cur]) {
        ans ~= cur;
    }
    ans ~= start;

    return ans;
}