#include <bits/stdc++.h>

using i32 = std::int32_t;
using u32 = std::uint32_t;
using i64 = std::int64_t;
using u64 = std::uint64_t;
using i128 = __int128_t;
using u128 = __uint128_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

class rep {
    struct Iter {
        usize itr;
        constexpr Iter(const usize pos) noexcept : itr(pos) {}
        constexpr void operator++() noexcept { ++itr; }
        constexpr bool operator!=(const Iter& other) const noexcept { return itr != other.itr; }
        constexpr usize operator*() const noexcept { return itr; }
    };
    const Iter first, last;

  public:
    explicit constexpr rep(const usize first, const usize last) noexcept : first(first), last(std::max(first, last)) {}
    constexpr Iter begin() const noexcept { return first; }
    constexpr Iter end() const noexcept { return last; }
};

template <class T> using Vec = std::vector<T>;

void main_() {
    usize N, M;
    std::cin >> N >> M;
    Vec<Vec<std::pair<usize, usize>>> graph(N);
    Vec<usize> idx(M);
    for (const auto i : rep(0, M)) {
        usize u, v;
        std::cin >> u >> v;
        char c;
        std::cin >> c;
        u -= 1;
        v -= 1;
        graph[u].emplace_back(v, i);
        graph[v].emplace_back(u, i);
        idx[i] = c - 'a';
    }
    if (std::count(idx.begin(), idx.end(), idx[0]) == (isize)M) {
        std::cout << -1 << '\n';
        return;
    }
    Vec<std::array<usize, 27>> last(N);
    Vec<std::array<std::pair<usize, usize>, 27>> position(N);
    for (auto& e : last) {
        e.fill(M);
    }
    std::queue<std::pair<usize, usize>> que;
    const auto push = [&](const usize u, const usize k, const usize a, const usize b, const usize c) {
        if (last[u][k] == M) {
            last[u][k] = a;
            position[u][k] = std::make_pair(b, c);
            que.emplace(u, k);
        }
    };
    for (const auto i : rep(0, 26)) {
        push(0, i, -1, 0, 0);
    }
    while (!que.empty()) {
        const auto [u, k] = que.front();
        que.pop();
        for (const auto& [v, i] : graph[u]) {
            if (last[u][k] == (usize)-1) {
                if (k == idx[i]) {
                    push(v, k, i, u, k);
                }
            } else {
                push(v, k == idx[i] ? k : 26, i, u, k);
            }
        }
    }
    Vec<usize> path;
    usize u = N - 1, k = 26;
    while (~last[u][k]) {
        path.push_back(last[u][k]);
        std::tie(u, k) = position[u][k];
    }
    std::reverse(path.begin(), path.end());
    bool pal = true;
    for (const auto i : rep(0, path.size())) {
        if (idx[path[i]] != idx[path[path.size() - i - 1]]) {
            pal = false;
        }
    }
    if (pal) {
        path.push_back(path.back());
        path.push_back(path.back());
    }
    std::cout << path.size() << '\n';
    for (const auto x : path) {
        std::cout << x + 1 << '\n';
    }
}

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);
    main_();
    return 0;
}