#include 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 using Vec = std::vector; void main_() { usize N, M; std::cin >> N >> M; Vec>> graph(N); Vec 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> last(N); Vec, 27>> position(N); for (auto& e : last) { e.fill(M); } std::queue> 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, -1, -1); } 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 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[i] != idx[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; }