#include <bits/stdc++.h>
using namespace std;
#define rep(i, x) for(int i = 0; i < (x); i++)
constexpr int inf = 1 << 30;
using T = tuple<int, int, int>;

int bit;
int main() {
	int n, m;
	cin >> n >> m;
	vector<T> edges;
	vector<vector<pair<int, int>>> g(n);

	auto is_palindrome = [](const string &s) {
		rep(i, s.size()) {
			if(s[i] != s[s.size() - 1 - i]) return false;
		}
		return true;
	};

	auto bfs = [&](int s) {
		vector dist(n, inf);
		dist[s] = 0;
		queue<int> que;
		que.push(s);
		while(que.size()) {
			int now = que.front();
			que.pop();
			for(auto [to, ignored] : g[now]) {
				if(dist[to] <= dist[now] + 1) continue;
				dist[to] = dist[now] + 1;
				que.push(to);
			}
		}
		return dist;
	};

	auto out = [](vector<int> &res) {
		reverse(res.begin(), res.end());
		cout << res.size() << endl;
		for(auto &v : res) cout << v + 1 << endl;
		exit(0);
	};

	rep(i, m) {
		int a, b;
		char c;
		cin >> a >> b >> c;
		--a, --b;
		g[a].emplace_back(b, i);
		g[b].emplace_back(a, i);
		edges.push_back({a, b, c - 'a'});
		bit |= 1 << (c - 'a');
	}
	if(__builtin_popcount(bit) == 1) {
		puts("-1");
		return 0;
	}

	auto dist = bfs(0);
	vector<int> res;
	string tmp = "";
	{
		int now = n - 1;
		while(now) {
			for(auto [to, id] : g[now]) {
				if(dist[to] + 1 == dist[now]) {
					now = to;
					tmp.push_back(get<2>(edges[id]));
					res.push_back(id);
					break;
				}
			}
		}
	}
	if(not is_palindrome(tmp)) {
		out(res);
	}
	if(count(tmp.begin(), tmp.end(), tmp.front()) != tmp.size()) {
		rep(i, 2) res.push_back(res.back());
		out(res);
	}
	rep(i, m) {
		auto [u, v, e] = edges[i];
		if(e == tmp.front()) continue;
		res.clear();
		tmp.clear();
		if(dist[u] > dist[v]) swap(u, v);
		//0からu u->v v->u uからn
		auto d2 = bfs(u);
		{
			int now = n - 1;
			while(now != u) {
				for(auto [to, id] : g[now]) {
					if(d2[to] + 1 == d2[now]) {
						now = to;
						tmp.push_back(get<2>(edges[id]));
						res.push_back(id);
						break;
					}
				}
			}
		}
		rep(j, 2) {
			tmp.push_back((char)('a' + e));
			res.push_back(i);
		}
		{
			int now = u;
			while(now) {
				for(auto [to, id] : g[now]) {
					if(dist[to] + 1 == dist[now]) {
						now = to;
						tmp.push_back(get<2>(edges[id]));
						res.push_back(id);
						break;
					}
				}
			}
		}
		break;
	}
	if(is_palindrome(tmp)) {
		rep(i, 2) res.push_back(res.back());
	}
	out(res);
}