Buggy python submission (verdict: Runtime Error):
from collections import deque

N,M = [int(v) for v in input().split()]
ad_list = [[] for _ in range (N+1)]
ad_num = [0 for _ in range(N)]
topo = []

for _ in range(M):
    l,r = [int(v) for v in input().split()]
    ad_list[l].append(r)
    ad_num[r-1] += 1

for i in range(1,N+1):
    if ad_num[i-1] == 0:
        topo.append(i)
queue = deque(topo[:])
while queue:
    n1 = queue.popleft()
    for n2 in ad_list[n1]:
        ad_num[n2-1] -= 1
        if ad_num[n2-1] == 0:
            topo.append(n2)
            queue.append(n2)

visited = [False for _ in range(N+1)]

ans = 0
def dfs(node,cnt=0):
    global ans
    ans = max(ans,cnt)
    if visited[node]: return 0
    visited[node] = True
    for n in ad_list[node]:
        dfs(n,cnt+1)

for n in topo:
    dfs(n)
print(ans)


Fixed submission:
from collections import deque

N,M = [int(v) for v in input().split()]
ad_list = [[] for _ in range (N+1)]
ad_num = [0 for _ in range(N)]
topo = []

for _ in range(M):
    l,r = [int(v) for v in input().split()]
    ad_list[l].append(r)
    ad_num[r-1] += 1

for i in range(1,N+1):
    if ad_num[i-1] == 0:
        topo.append([i,0])
queue = deque(topo[:])
while queue:
    n1,cnt = queue.popleft()
    for n2 in ad_list[n1]:
        ad_num[n2-1] -= 1
        if ad_num[n2-1] == 0:
            topo.append([n2,cnt+1])
            queue.append([n2,cnt+1])
print(max([v[1] for v in topo]))

Buggy python submission (verdict: Runtime Error):
from collections import deque

N,M = [int(v) for v in input().split()]
ad_list = [[] for _ in range (N+1)]
ad_num = [0 for _ in range(N)]
topo = []

for _ in range(M):
    l,r = [int(v) for v in input().split()]
    ad_list[l].append(r)
    ad_num[r-1] += 1

for i in range(1,N+1):
    if ad_num[i-1] == 0:
        topo.append(i)
queue = deque(topo[:])
while queue:
    n1 = queue.popleft()
    for n2 in ad_list[n1]:
        ad_num[n2-1] -= 1
        if ad_num[n2-1] == 0:
            topo.append(n2)
            queue.append(n2)
visited = [False for _ in range(N+1)]

ans = 0
path_len = 0
def dfs(node,cnt=0):
    global ans
    if visited[node]: return 0
    visited[node] = True
    ans = max(ans,cnt)
    for n in ad_list[node]:
        dfs(n,cnt+1)
    visited[node] = False

for n in topo:
    dfs(n)
print(ans)


Fixed submission:
from collections import deque

N,M = [int(v) for v in input().split()]
ad_list = [[] for _ in range (N+1)]
ad_num = [0 for _ in range(N)]
topo = []

for _ in range(M):
    l,r = [int(v) for v in input().split()]
    ad_list[l].append(r)
    ad_num[r-1] += 1

for i in range(1,N+1):
    if ad_num[i-1] == 0:
        topo.append([i,0])
queue = deque(topo[:])
while queue:
    n1,cnt = queue.popleft()
    for n2 in ad_list[n1]:
        ad_num[n2-1] -= 1
        if ad_num[n2-1] == 0:
            topo.append([n2,cnt+1])
            queue.append([n2,cnt+1])
print(max([v[1] for v in topo]))

Buggy python submission (verdict: Runtime Error):
from collections import deque

N,M = [int(v) for v in input().split()]
ad_list = [[] for _ in range (N+1)]
ad_num = [0 for _ in range(N)]
topo = []

for _ in range(N-1+M):
    l,r = [int(v) for v in input().split()]
    ad_list[l].append(r)
    ad_num[r-1] += 1

for i in range(1,N+1):
    if ad_num[i-1] == 0:
        topo.append(i)
queue = deque(topo[:])
topo = [[0,topo[0]]]
while queue:
    n1 = queue.popleft()
    for n2 in ad_list[n1]:
        ad_num[n2-1] -= 1
        if ad_num[n2-1] == 0:
            topo.append([n1,n2])
            queue.append(n2)
topo.sort(key=lambda x:x[1])
for i,v in topo:
    print(i)
#  visited = [False for _ in range(N)]

#  ans = 0
#  path_len = 0
#  def dfs(node,cnt=1):
    #  global ans
    #  if visited[node-1]: return 0
    #  visited[node-1] = True
    #  ans = max(ans,cnt)
    #  for n in ad_list[node]:
        #  dfs(n,cnt+1)

#  for n in range(1,N+1):
    #  dfs(n)
#  print(ans)


Fixed submission:
from collections import deque

N,M = [int(v) for v in input().split()]
ad_list = [[] for _ in range (N+1)]
ad_num = [0 for _ in range(N)]
topo = []

for _ in range(M):
    l,r = [int(v) for v in input().split()]
    ad_list[l].append(r)
    ad_num[r-1] += 1

for i in range(1,N+1):
    if ad_num[i-1] == 0:
        topo.append([i,0])
queue = deque(topo[:])
while queue:
    n1,cnt = queue.popleft()
    for n2 in ad_list[n1]:
        ad_num[n2-1] -= 1
        if ad_num[n2-1] == 0:
            topo.append([n2,cnt+1])
            queue.append([n2,cnt+1])
print(max([v[1] for v in topo]))

Buggy python submission (verdict: Runtime Error):
N,M = [int(v) for v in input().split()]
ad_list = [[] for _ in range (N+1)]
ad_num = [0 for _ in range(N)]
topo = []

for _ in range(M):
    l,r = [int(v) for v in input().split()]
    ad_list[l].append(r)
    ad_num[r-1] += 1
for i in range(1,N+1):
    if ad_num[i-1] == 0:
        topo.append(i)

queue = topo[:]
while queue:
    n1 = queue.pop(0)
    for n2 in ad_list[n1]:
        ad_num[n2-1] -= 1
        if ad_num[n2-1] == 0:
            topo.append(n2)
            queue.append(n2)

visited = [False for _ in range(N)]

ans = 0
path_len = 0
def dfs(node,cnt=1):
    global ans
    if visited[node-1]: return 0
    visited[node-1] = True
    ans = max(ans,cnt)
    for n in ad_list[node]:
        dfs(n,cnt+1)

for n in range(1,N+1):
    dfs(n)
print(ans)


Fixed submission:
from collections import deque

N,M = [int(v) for v in input().split()]
ad_list = [[] for _ in range (N+1)]
ad_num = [0 for _ in range(N)]
topo = []

for _ in range(M):
    l,r = [int(v) for v in input().split()]
    ad_list[l].append(r)
    ad_num[r-1] += 1

for i in range(1,N+1):
    if ad_num[i-1] == 0:
        topo.append([i,0])
queue = deque(topo[:])
while queue:
    n1,cnt = queue.popleft()
    for n2 in ad_list[n1]:
        ad_num[n2-1] -= 1
        if ad_num[n2-1] == 0:
            topo.append([n2,cnt+1])
            queue.append([n2,cnt+1])
print(max([v[1] for v in topo]))