import sys

sys.setrecursionlimit(1000006)
# sys.set_int_max_str_digits(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
# inf = -1-(-1 << 31)
inf = -1-(-1 << 62)

# md = 10**9+7
md = 998244353

def dfs(ok,u):
    if ok(u):return u
    for v,i in to[u]:
        if fin[v]:continue
        fin[v]=1
        ii.append(i)
        ret=dfs(ok,v)
        if ret!=-1:return ret
        ii.pop()
        fin[v]=0
    return -1

n,m=LI()
to=[[] for _ in range(n)]
cc=[]
for i in range(m):
    u,v,c=SI().split()
    u=int1(u)
    v=int1(v)
    c=ord(c)-97
    to[u].append((v,i))
    to[v].append((u,i))
    cc.append(c)

if all(c==cc[0] for c in cc):
    print(-1)
    exit()
 
s,si=to[0][0]
ii=[si]
fin=[0]*n
fin[s]=1
ok1=lambda x:cc[ii[-1]]!=cc[si]
v=dfs(ok1,s)
# print(ii,v)

if v!=n-1:
    pre,ii=ii,[]
    fin=[0]*n
    fin[v]=1
    ok2=lambda x:x==n-1
    dfs(ok2,v)
    ii=pre+ii

# print(ii)
ee=[cc[i] for i in ii]
if ee==ee[::-1]:ii=[si,si]+ii
print(len(ii))
for i in ii:print(i+1)