from collections import deque

n, m = map(int, input().split())
to = [[] for _ in range(n)]
for i in range(1, m + 1):
    a, b, c = input().split()
    a = int(a) - 1
    b = int(b) - 1
    c = ord(c) - 97
    to[a].append((i, b, c))
    to[b].append((i, a, c))

z = -1
for i in range(n):
    seen = 0
    for _, _, c in to[i]:
        seen |= 1 << c
    if bin(seen).count('1') >= 2:
        z = i
        break

if z == -1:
    print(-1)
    exit()

INF = 10 ** 9
def shortest_path(s, t):
    pre = [None] * n
    dist = [INF] * n
    q = deque()
    dist[s] = 0
    q.append(s)
    while q:
        u = q.popleft()
        for i, v, c in to[u]:
            if dist[v] == INF:
                dist[v] = dist[u] + 1
                pre[v] = (u, i, c)
                q.append(v)
    u = t
    path = []
    s = []
    while pre[u] is not None:
        u, i, c = pre[u]
        path.append(i)
        s.append(c)
    s.reverse()
    path.reverse()
    return path, s

p1, s1 = shortest_path(0, z)
p2, s2 = shortest_path(n - 1, z)
s = s1 + s2[::-1]
if s == s[::-1]:
    if s1 == s2:
        last_edge = p1[-1]
        last_col = s1[-1]
        for i, _, c in to[z]:
            if c != last_col:
                break
        p1 += last_edge, last_edge, i, i
    else:
        seen = [False] * 26
        for i, _, c in to[z]:
            if seen[c]:
                continue
            seen[c] = True
            t = s1 + [c, c] + s2[::-1]
            if t != t[::-1]:
                p1 += i, i
                break

p = p1 + p2[::-1]
print(len(p))
print(*p, sep='\n')