Buggy python submission (verdict: Time Limit Exceeded):
import os
import sys

import numpy as np
from scipy.sparse import csr_matrix
from scipy.sparse.csgraph import dijkstra

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

U = []
V = []
for u, v in UV:
    U.extend([u, u + N, u + N * 2])
    V.extend([v + N, v + N * 2, v])

graph = csr_matrix(([1] * (M * 3), (U, V)), shape=(N * 3 + 1, N * 3 + 1))
dist = dijkstra(graph, indices=S)[T]
if dist < np.inf:
    print(int(dist // 3))
else:
    print(-1)


Fixed submission:
import bisect
import heapq
import itertools
import math
import os
import re
import string
import sys
from collections import Counter, deque, defaultdict
from decimal import Decimal
from fractions import gcd
from functools import lru_cache, reduce
from operator import itemgetter

import numpy as np

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = [[] for _ in range(N + 1)]
for u, v in UV:
    graph[u].append(v)

que = deque()
que.append((S, 0))
reached = np.zeros((N + 1, 3), dtype=bool)
reached[S, 0] = True

while que:
    u, step = que.popleft()
    for v in graph[u]:
        s = step + 1
        if not reached[v, s % 3]:
            reached[v, s % 3] = True
            que.append((v, s))
        if v == T and s % 3 == 0:
            print(s // 3)
            exit()
print(-1)

Buggy python submission (verdict: Time Limit Exceeded):
import os
import sys
from collections import defaultdict

from functools import lru_cache

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = defaultdict(set)
for u, v in UV:
    graph[u].add(v)

vs = {S}
checked = set()
i = 0
prev = 0
while True:
    i += 1
    checked |= vs
    for _ in range(3):
        ss = set()
        for v in vs:
            ss |= graph[v]
        vs = ss
    if T in vs:
        print(i)
        break
    if len(checked) == prev:
        print(-1)
        break
    prev = len(checked)


Fixed submission:
import bisect
import heapq
import itertools
import math
import os
import re
import string
import sys
from collections import Counter, deque, defaultdict
from decimal import Decimal
from fractions import gcd
from functools import lru_cache, reduce
from operator import itemgetter

import numpy as np

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = [[] for _ in range(N + 1)]
for u, v in UV:
    graph[u].append(v)

que = deque()
que.append((S, 0))
reached = np.zeros((N + 1, 3), dtype=bool)
reached[S, 0] = True

while que:
    u, step = que.popleft()
    for v in graph[u]:
        s = step + 1
        if not reached[v, s % 3]:
            reached[v, s % 3] = True
            que.append((v, s))
        if v == T and s % 3 == 0:
            print(s // 3)
            exit()
print(-1)

Buggy python submission (verdict: Runtime Error):
import os
import sys
from collections import defaultdict

import numpy as np
from functools import lru_cache

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = np.zeros((N + 1, N + 1), dtype=bool)
for u, v in UV:
    graph[u][v] = True


@lru_cache(maxsize=None)
def dest(v, count):
    if count <= 1:
        return graph[v]
    ret = np.zeros((N + 1), dtype=bool)
    for u in np.where(graph[v])[0]:
        ret |= dest(u, count - 1)
    return ret


checked = np.zeros(N + 1, dtype=bool)
vs = np.zeros(N + 1, dtype=bool)
vs[S] = True
i = 0
prev = 0
while True:
    i += 1
    s = np.zeros(N + 1, dtype=bool)
    for v in np.where(vs)[0]:
        s |= dest(v, 3)
    checked |= vs
    if s[T]:
        print(i)
        break
    if checked.sum() == prev:
        print(-1)
        break
    vs = s ^ checked
    prev = checked.sum()


Fixed submission:
import bisect
import heapq
import itertools
import math
import os
import re
import string
import sys
from collections import Counter, deque, defaultdict
from decimal import Decimal
from fractions import gcd
from functools import lru_cache, reduce
from operator import itemgetter

import numpy as np

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = [[] for _ in range(N + 1)]
for u, v in UV:
    graph[u].append(v)

que = deque()
que.append((S, 0))
reached = np.zeros((N + 1, 3), dtype=bool)
reached[S, 0] = True

while que:
    u, step = que.popleft()
    for v in graph[u]:
        s = step + 1
        if not reached[v, s % 3]:
            reached[v, s % 3] = True
            que.append((v, s))
        if v == T and s % 3 == 0:
            print(s // 3)
            exit()
print(-1)

Buggy python submission (verdict: Time Limit Exceeded):
import os
import sys
from collections import defaultdict

from functools import lru_cache

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = defaultdict(set)
for u, v in UV:
    graph[u].add(v)

memo = [[None] * (N+1) for _ in range(3)]


def dest(v, count):
    if memo[count - 1][v] is not None:
        return memo[count - 1][v]

    if count <= 1:
        memo[count - 1][v] = graph[v]
        return graph[v]
    ret = set()
    for u in graph[v]:
        ret |= dest(u, count - 1)
    memo[count - 1][v] = ret
    return ret


checked = set()
vs = {S}
i = 0
prev = 0
while True:
    i += 1
    s = set()
    for v in vs:
        s |= dest(v, 3)
    checked |= vs
    if T in s:
        print(i)
        break
    if len(checked) == prev:
        print(-1)
        break
    vs = s - checked
    prev = len(checked)


Fixed submission:
import bisect
import heapq
import itertools
import math
import os
import re
import string
import sys
from collections import Counter, deque, defaultdict
from decimal import Decimal
from fractions import gcd
from functools import lru_cache, reduce
from operator import itemgetter

import numpy as np

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = [[] for _ in range(N + 1)]
for u, v in UV:
    graph[u].append(v)

que = deque()
que.append((S, 0))
reached = np.zeros((N + 1, 3), dtype=bool)
reached[S, 0] = True

while que:
    u, step = que.popleft()
    for v in graph[u]:
        s = step + 1
        if not reached[v, s % 3]:
            reached[v, s % 3] = True
            que.append((v, s))
        if v == T and s % 3 == 0:
            print(s // 3)
            exit()
print(-1)

Buggy python submission (verdict: Time Limit Exceeded):
import os
import sys
from collections import defaultdict

from functools import lru_cache

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = defaultdict(set)
for u, v in UV:
    graph[u].add(v)


@lru_cache(maxsize=None)
def dest(v, count):
    if count <= 1:
        return graph[v]
    ret = set()
    for u in graph[v]:
        ret |= dest(u, count - 1)
    return ret


checked = set()
vs = {S}
i = 0
prev = 0
while True:
    i += 1
    s = set()
    for v in vs:
        s |= dest(v, 3)
    checked |= vs
    if T in s:
        print(i)
        break
    if len(checked) == prev:
        print(-1)
        break
    vs = s - checked
    prev = len(checked)


Fixed submission:
import bisect
import heapq
import itertools
import math
import os
import re
import string
import sys
from collections import Counter, deque, defaultdict
from decimal import Decimal
from fractions import gcd
from functools import lru_cache, reduce
from operator import itemgetter

import numpy as np

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = [[] for _ in range(N + 1)]
for u, v in UV:
    graph[u].append(v)

que = deque()
que.append((S, 0))
reached = np.zeros((N + 1, 3), dtype=bool)
reached[S, 0] = True

while que:
    u, step = que.popleft()
    for v in graph[u]:
        s = step + 1
        if not reached[v, s % 3]:
            reached[v, s % 3] = True
            que.append((v, s))
        if v == T and s % 3 == 0:
            print(s // 3)
            exit()
print(-1)

Buggy python submission (verdict: Time Limit Exceeded):
import os
import sys
from collections import defaultdict

from functools import lru_cache

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = defaultdict(set)
for u, v in UV:
    graph[u].add(v)


@lru_cache(maxsize=None)
def dest(v, count):
    if count <= 1:
        return graph[v]
    ret = set()
    for u in graph[v]:
        ret |= dest(u, count - 1)
    return ret


checked = set()
vs = {S}
i = 0
prev = 0
while True:
    i += 1
    s = set()
    for v in vs:
        if v not in checked:
            s |= dest(v, 3)
    checked |= vs
    if T in s:
        print(i)
        break
    if len(checked) == prev:
        print(-1)
        break
    vs = s
    prev = len(checked)


Fixed submission:
import bisect
import heapq
import itertools
import math
import os
import re
import string
import sys
from collections import Counter, deque, defaultdict
from decimal import Decimal
from fractions import gcd
from functools import lru_cache, reduce
from operator import itemgetter

import numpy as np

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = [[] for _ in range(N + 1)]
for u, v in UV:
    graph[u].append(v)

que = deque()
que.append((S, 0))
reached = np.zeros((N + 1, 3), dtype=bool)
reached[S, 0] = True

while que:
    u, step = que.popleft()
    for v in graph[u]:
        s = step + 1
        if not reached[v, s % 3]:
            reached[v, s % 3] = True
            que.append((v, s))
        if v == T and s % 3 == 0:
            print(s // 3)
            exit()
print(-1)

Buggy python submission (verdict: Time Limit Exceeded):
import os
import sys
from collections import defaultdict

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

# 3 つ先の頂点同士を結んだグラフを作って最短距離

graph = defaultdict(set)
for u, v in UV:
    graph[u].add(v)

v1 = graph.copy()
# 2回で行ける
v2 = defaultdict(set)
for u, vs in graph.items():
    for v in vs:
        v2[u] |= v1[v]
# 3回
v3 = defaultdict(set)
for u, vs in v2.items():
    for v in vs:
        v3[u] |= v1[v]

fixed = {S}
vs = {S}
i = 0
prev = 1
while True:
    i += 1
    s = set()
    for v in vs:
        s |= v3[v]
    if T in s:
        print(i)
        break
    fixed |= s
    if len(fixed) == prev:
        print(-1)
        break
    vs = s
    prev = len(fixed)


Fixed submission:
import bisect
import heapq
import itertools
import math
import os
import re
import string
import sys
from collections import Counter, deque, defaultdict
from decimal import Decimal
from fractions import gcd
from functools import lru_cache, reduce
from operator import itemgetter

import numpy as np

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")
IINF = 10 ** 18

N, M = list(map(int, sys.stdin.readline().split()))
UV = [list(map(int, sys.stdin.readline().split())) for _ in range(M)]
S, T = list(map(int, sys.stdin.readline().split()))

graph = [[] for _ in range(N + 1)]
for u, v in UV:
    graph[u].append(v)

que = deque()
que.append((S, 0))
reached = np.zeros((N + 1, 3), dtype=bool)
reached[S, 0] = True

while que:
    u, step = que.popleft()
    for v in graph[u]:
        s = step + 1
        if not reached[v, s % 3]:
            reached[v, s % 3] = True
            que.append((v, s))
        if v == T and s % 3 == 0:
            print(s // 3)
            exit()
print(-1)