#include <stdio.h>

typedef struct List {
	struct List *next;
	int v, id;
} list;

int is_palindrome(char S[])
{
	int i, j;
	for (j = 0; S[j] != 0; j++);
	for (i = 0, j--; i < j; i++, j--) if (S[i] != S[j]) break;
	if (i < j) return 0;
	else return 1;
}

int is_unique(char S[])
{
	int i;
	for (i = 1; S[i] != 0; i++) if (S[i] != S[0]) break;
	if (S[i] != 0) return 0;
	else return 1;
}

int solve(int N, int M, list* adj[], char c[], int ans[])
{
	int i, j, k, u, w, par[2][100001] = {}, q[100001], head, tail;
	list *p;
	for (i = 2; i <= M; i++) if (c[i] != c[1]) break;
	if (i > M) return -1;
	par[0][N] = -N;
	q[0] = N;
	for (head = 0, tail = 1; head < tail; head++) {
		u = q[head];
		for (p = adj[u]; p != NULL; p = p->next) {
			w = p->v;
			if (par[0][w] == 0) {
				par[0][w] = u;
				par[1][w] = p->id;
				q[tail++] = w;
			}
		}
	}
	if (par[0][1] == 0) return -1;
	
    char S[200001], T[200001];
	for (u = 1, k = 0; u != N; u = -par[0][u], k++) {
		S[k] = c[par[1][u]];
		ans[k] = par[1][u];
		q[k] = u;
		par[0][u] *= -1;
	}
	S[k] = 0;
	if (is_palindrome(S) == 0) return k;
	else if (is_unique(S) == 0) {
		ans[k] = ans[k-1];
		ans[k+1] = ans[k];
		return k + 2;
	}
	
	q[k++] = N;
	for (u = 1; u < N; u++) if (par[0][u] > 0) par[0][u] = 0;
	for (head = 0, tail = k; head < tail; head++) {
		u = q[head];
		for (p = adj[u]; p != NULL; p = p->next) {
			w = p->v;
			if (c[p->id] != S[0]) break;
			else if (par[0][w] == 0) {
				par[0][w] = u;
				par[1][w] = p->id;
				q[tail++] = w;
			}
		}
		if (p != NULL) break;
	}
	if (head == tail) return -1;
	
	int tmp[200001];
	T[0] = c[p->id];
	tmp[0] = p->id;
	for (j = 1; par[0][u] > 0; u = par[0][u], j++) {
		T[j] = c[par[1][u]];
		tmp[j] = par[1][u];
	}
	for (w = u, u = 1, k = 0; u != w; u = -par[0][u], k++) {
		S[k] = c[par[1][u]];
		ans[k] = par[1][u];
	}
	for (i = j - 1; i >= 0; i--, k++) {
		S[k] = T[i];
		ans[k] = tmp[i];
	}
	for (i = 0; i < j; i++, k++) {
		S[k] = T[i];
		ans[k] = tmp[i];
	}
	for (; u != N; u = -par[0][u], k++) {
		S[k] = c[par[1][u]];
		ans[k] = par[1][u];
	}
	S[k] = 0;
	if (is_palindrome(S) == 0) return k;
	else {
		ans[k] = ans[k-1];
		ans[k+1] = ans[k];
		return k + 2;
	}
}

int main()
{
	int i, N, M, u, w;
	char c[200001];
    list *adj[100001] = {}, e[400001];
	scanf("%d %d", &N, &M);
	for (i = 0; i < M; i++) {
		scanf("%d %d %c ", &u, &w, &(c[i+1]));
		e[i*2].v = w;
		e[i*2+1].v = u;
		e[i*2].id = i + 1;
		e[i*2+1].id = i + 1;
		e[i*2].next = adj[u];
		e[i*2+1].next = adj[w];
		adj[u] = &(e[i*2]);
		adj[w] = &(e[i*2+1]);
	}
	
	int ans[200001], k = solve(N, M, adj, c, ans);
	printf("%d\n", k);
	if (k >= 0) for (i = 0; i < k; i++) printf("%d\n", ans[i]);
	fflush(stdout);
	return 0;
}