Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(100000)

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

if 0 == M:
	print(-1)
	exit()

s, t = S - 1, T - 1

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

INF = 10 ** 12
cost = [[INF, INF, INF] for _ in range(N)]
def print_cost(cost):
	for line in cost:
		for cell in line:
			print("  INF" if cell == INF else " {:4d}".format(cell), end="")
		print()
	print()

def dfs(graph, cost, c, d):
	if cost[c][d % 3] <= d:
		return
	cost[c][d % 3] = d
	#print_cost(cost)
	for n in graph[c]:
		dfs(graph, cost, n, d + 1)

dfs(graph, cost, s, 0)

if INF == cost[t][0]:
	print(-1)
else:
	print(cost[t][0] // 3)


Fixed submission:
N, M = map(int, input().split())
UV = [list(map(int, input().split())) for _ in range(M)]
S, T = map(int, input().split())

if 0 == M:
	print(-1)
	exit()

graph = [set() for _ in range(N)]
for u, v in UV:
	graph[u - 1] |= {v - 1}

INF = 10 ** 12
depths = [[INF, INF, INF] for _ in range(N)]

def bfs(start, end):
	depth = 0
	nodes = {start}
	while nodes:
		if (0 == depth % 3) and (end in nodes):
			return depth // 3
		next_nodes = set()
		for node in nodes:
			if depths[node][depth % 3] <= depth:
				continue
			depths[node][depth % 3] = depth
			next_nodes |= graph[node]
		nodes = next_nodes
		depth += 1
	return -1

print(bfs(S - 1, T - 1))

Buggy python submission (verdict: Runtime Error):
N, M = map(int, input().split())
UV = [list(map(int, input().split())) for _ in range(M)]
S, T = map(int, input().split())

if 0 == M:
	print(-1)
	exit()

s, t = S - 1, T - 1

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

INF = 10 ** 12
cost = [[INF, INF, INF] for _ in range(N)]
def print_cost(cost):
	for line in cost:
		for cell in line:
			print("  INF" if cell == INF else " {:4d}".format(cell), end="")
		print()
	print()

def dfs(graph, cost, c, d):
	if cost[c][d % 3] <= d:
		return
	cost[c][d % 3] = d
	#print_cost(cost)
	for n in graph[c]:
		dfs(graph, cost, n, d + 1)

dfs(graph, cost, s, 0)

print(cost[t][0] // 3 if 0 == cost[t][0] % 3 else -1)


Fixed submission:
N, M = map(int, input().split())
UV = [list(map(int, input().split())) for _ in range(M)]
S, T = map(int, input().split())

if 0 == M:
	print(-1)
	exit()

graph = [set() for _ in range(N)]
for u, v in UV:
	graph[u - 1] |= {v - 1}

INF = 10 ** 12
depths = [[INF, INF, INF] for _ in range(N)]

def bfs(start, end):
	depth = 0
	nodes = {start}
	while nodes:
		if (0 == depth % 3) and (end in nodes):
			return depth // 3
		next_nodes = set()
		for node in nodes:
			if depths[node][depth % 3] <= depth:
				continue
			depths[node][depth % 3] = depth
			next_nodes |= graph[node]
		nodes = next_nodes
		depth += 1
	return -1

print(bfs(S - 1, T - 1))

Buggy python submission (verdict: Runtime Error):
N, M = map(int, input().split())
UV = [list(map(int, input().split())) for _ in range(M)]
S, T = map(int, input().split())

if 0 == M:
	print(-1)
	exit()

s, t = S - 1, T - 1

graph = [[] for _ in range(M)]
for u, v in UV:
	graph[u - 1] += [v - 1]

INF = 10 ** 12
cost = [[INF, INF, INF] for _ in range(M)]
def print_cost(cost):
	for line in cost:
		for cell in line:
			print("  INF" if cell == INF else " {:4d}".format(cell), end="")
		print()
	print()

def dfs(graph, cost, c, d):
	if cost[c][d % 3] <= d:
		return
	cost[c][d % 3] = d
	#print_cost(cost)
	for n in graph[c]:
		dfs(graph, cost, n, d + 1)

dfs(graph, cost, 0, 0)

print(cost[T - 1][0] // 3 if 0 == cost[T - 1][0] % 3 else -1)


Fixed submission:
N, M = map(int, input().split())
UV = [list(map(int, input().split())) for _ in range(M)]
S, T = map(int, input().split())

if 0 == M:
	print(-1)
	exit()

graph = [set() for _ in range(N)]
for u, v in UV:
	graph[u - 1] |= {v - 1}

INF = 10 ** 12
depths = [[INF, INF, INF] for _ in range(N)]

def bfs(start, end):
	depth = 0
	nodes = {start}
	while nodes:
		if (0 == depth % 3) and (end in nodes):
			return depth // 3
		next_nodes = set()
		for node in nodes:
			if depths[node][depth % 3] <= depth:
				continue
			depths[node][depth % 3] = depth
			next_nodes |= graph[node]
		nodes = next_nodes
		depth += 1
	return -1

print(bfs(S - 1, T - 1))