Buggy python submission (verdict: Time Limit Exceeded):
# 最大でも3Nしか頂点には回らない
import sys
from collections import deque
input = sys.stdin.readline
n, m = map(int, input().split())

edges = [[] for i in range(n)]
for i in range(m):
    u,v = map(int, input().split())
    u -= 1
    v -= 1
    edges[u].append(v)

dist = [[False]*3 for i in range(n)]
cnt = [[0]*3 for i in range(n)]
s, t = map(int, input().split())
s -= 1
t -= 1
q = deque()
# now, dist(mod3), cnt
q.append((s, 0, 0))
while q:
    now, d, c = q.popleft()
    if dist[now][d] and cnt[now][d] < c:
        continue
    dist[now][d] = True
    cnt[now][d] = c
    next_d = (d+1)%3
    next_c = c+1
    for to in edges[now]:
        if not dist[to][next_d] or cnt[to][next_d] > c:
            q.appendleft((to, next_d,next_c))
if dist[t][0]:
    print(cnt[t][0]//3)
else:
    print(-1)

Fixed submission:
# 最大でも3Nしか頂点には回らない
import sys
from collections import deque
input = sys.stdin.readline
n, m = map(int, input().split())

edges = [[] for i in range(n)]
for i in range(m):
    u,v = map(int, input().split())
    u -= 1
    v -= 1
    edges[u].append(v)

dist = [[False]*3 for i in range(n)]
cnt = [[0]*3 for i in range(n)]
s, t = map(int, input().split())
s -= 1
t -= 1
q = deque()
# now, dist(mod3), cnt
q.append((s, 0, 0))
while q:
    now, d, c = q.popleft()
    if dist[now][d] and cnt[now][d] <= c:
        continue
    dist[now][d] = True
    cnt[now][d] = c
    next_d = (d+1)%3
    next_c = c+1
    for to in edges[now]:
        if not dist[to][next_d] or cnt[to][next_d] > c:
            q.append((to, next_d,next_c))
if dist[t][0]:
    print(cnt[t][0]//3)
else:
    print(-1)

Buggy python submission (verdict: Time Limit Exceeded):
# 最大でも3Nしか頂点には回らない
import sys
from collections import deque
input = sys.stdin.readline
n, m = map(int, input().split())

edges = [[] for i in range(n)]
for i in range(m):
    u,v = map(int, input().split())
    u -= 1
    v -= 1
    edges[u].append(v)

dist = [[False]*3 for i in range(n)]
cnt = [[0]*3 for i in range(n)]
s, t = map(int, input().split())
s -= 1
t -= 1
q = deque()
# now, dist(mod3), cnt
q.append((s, 0, 0))
while q:
    now, d, c = q.popleft()
    if dist[now][d] and cnt[now][d] < c:
        continue
    dist[now][d] = True
    cnt[now][d] = c
    next_d = (d+1)%3
    next_c = c+1
    for to in edges[now]:
        if not dist[to][next_d] or cnt[to][next_d] > c:
            q.append((to, next_d,next_c))
if dist[t][0]:
    print(cnt[t][0]//3)
else:
    print(-1)

Fixed submission:
# 最大でも3Nしか頂点には回らない
import sys
from collections import deque
input = sys.stdin.readline
n, m = map(int, input().split())

edges = [[] for i in range(n)]
for i in range(m):
    u,v = map(int, input().split())
    u -= 1
    v -= 1
    edges[u].append(v)

dist = [[False]*3 for i in range(n)]
cnt = [[0]*3 for i in range(n)]
s, t = map(int, input().split())
s -= 1
t -= 1
q = deque()
# now, dist(mod3), cnt
q.append((s, 0, 0))
while q:
    now, d, c = q.popleft()
    if dist[now][d] and cnt[now][d] <= c:
        continue
    dist[now][d] = True
    cnt[now][d] = c
    next_d = (d+1)%3
    next_c = c+1
    for to in edges[now]:
        if not dist[to][next_d] or cnt[to][next_d] > c:
            q.append((to, next_d,next_c))
if dist[t][0]:
    print(cnt[t][0]//3)
else:
    print(-1)