import sys
from collections import deque
sys.setrecursionlimit(10**7)
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


N,M = MI()
Graph = [[] for _ in range(N+1)]
edges = []
char = set()
for i in range(1,M+1):
    A,B,C = S().split()
    A = int(A)
    B = int(B)
    Graph[A].append((B,C,i))
    Graph[B].append((A,C,i))
    edges.append((A,B,C))
    char.add(C)

if len(char) == 1:
    print(-1)
    exit()

s,a,idx = Graph[1][0]

for i in range(M):
    A,B,C = edges[i]
    if a != C:
        goal = A
        e = i+1
        goal2 = B

inf = 10**18
dist = [inf]*(N+1)
dist[N] = (0,-1,-1)
deq = deque([N])
while deq:
    u = deq.pop()
    if u == goal:
        break
    for v,_,i in Graph[u]:
        if dist[v] == inf:
            dist[v] = (dist[u][0]+1,i,u)
            deq.appendleft(v)

ANS1 = [e]
ss = goal
while ss != N:
    d,i,u = dist[ss]
    ANS1.append(i)
    ss = u

ANS0 = [idx]*len(ANS1)
if len(ANS0) % 2 == 1:
    cur = s
else:
    cur = 1

dist = [inf]*(N+1)
dist[B] = (0,-1,-1)
deq = deque([B])
while deq:
    u = deq.pop()
    if u == cur:
        break
    for v,_,i in Graph[u]:
        if dist[v] == inf:
            dist[v] = (dist[u][0]+1,i,u)
            deq.appendleft(v)

s = cur
while s != B:
    d,i,u = dist[s]
    ANS0.append(i)
    s = u

ANS = ANS0+ANS1
print(len(ANS))
print(*ANS,sep='\n')