#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 n, m, bit, s, x, y, xy_id;
char sc;
string str;
int main() {
	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 = [&](const 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 restore = [&](vector<int> &vec, const int s, const int t) {
		vector<int> res;
		auto dist = bfs(s);
		int now = t;
		while(now != s) {
			for(auto [to, id] : g[now]) {
				if(dist[to] + 1 == dist[now]) {
					now = to;
					res.push_back(id);
					break;
				}
			}
		}
		reverse(res.begin(), res.end());
		vec.insert(vec.end(), res.begin(), res.end());
	};

	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;
	}

	//方針
	//0-s s-x x-y y-x x-(n-1) の順にパスを求める
	vector<int> res;
	rep(i, m) {
		auto [u, v, ig] = edges[i];
		if(!u or !v) {
			s = max(u, v);
			res.push_back(i);
			sc = get<2>(edges[i]);
			break;
		}
	}
	rep(i, m) {
		auto [u, v, c] = edges[i];
		if(c != sc) {
			auto dist = bfs(s);
			if(dist[u] > dist[v]) swap(u, v);
			x = u, y = v, xy_id = i;
			break;
		}
	}

	restore(res, s, x);
	rep(i, 2) res.push_back(xy_id);
	restore(res, x, n - 1);

	for(auto v : res) str.push_back(get<2>(edges[v]));
	if(is_palindrome(str)) rep(i, 2) res.push_back(res.back());

	cout << res.size() << endl;
	for(auto &v : res) cout << v + 1 << endl;

	return 0;
}