Buggy python submission (verdict: Runtime Error):
from collections import defaultdict
import sys
sys.setrecursionlimit(10**10)
N,M = map(int,input().split())

d = defaultdict(lambda:[])

for _ in range(M):
    x,y = map(int,input().split())
    x -= 1
    y -= 1
    d[x].append(y)

dp = [-1]*N

def dist(s):
    if dp[s] >= 0:
        return dp[s]
    if d[s] == []:
        dp[s] = 0
        return 0
    res = 0
    for t in d[s]:
        res = max(res,1+dist(t))
    dp[s] = res
    return res

ans = 0
for i in range(N):
    ans = max(ans,dist(i))

print(ans)

Fixed submission:
from collections import defaultdict
import sys
sys.setrecursionlimit(10**9)
N,M = map(int,input().split())

d = defaultdict(lambda:[])

for _ in range(M):
    x,y = map(int,input().split())
    x -= 1
    y -= 1
    d[x].append(y)

dp = [-1]*N

def dist(s):
    if dp[s] >= 0:
        return dp[s]
    if d[s] == []:
        dp[s] = 0
        return 0
    res = 0
    for t in d[s]:
        res = max(res,1+dist(t))
    dp[s] = res
    return res

ans = 0
for i in range(N):
    ans = max(ans,dist(i))

print(ans)

Buggy python submission (verdict: Runtime Error):
from collections import defaultdict
N,M = map(int,input().split())

d = defaultdict(lambda:[])

for _ in range(M):
    x,y = map(int,input().split())
    x -= 1
    y -= 1
    d[x].append(y)

dp = [-1]*N

def dist(s):
    if dp[s] >= 0:
        return dp[s]
    if d[s] == []:
        dp[s] = 0
        return 0
    res = 0
    for t in d[s]:
        res = max(res,1+dist(t))
    dp[s] = res
    return res

ans = 0
for i in range(N):
    ans = max(ans,dist(i))

print(ans)

Fixed submission:
from collections import defaultdict
import sys
sys.setrecursionlimit(10**9)
N,M = map(int,input().split())

d = defaultdict(lambda:[])

for _ in range(M):
    x,y = map(int,input().split())
    x -= 1
    y -= 1
    d[x].append(y)

dp = [-1]*N

def dist(s):
    if dp[s] >= 0:
        return dp[s]
    if d[s] == []:
        dp[s] = 0
        return 0
    res = 0
    for t in d[s]:
        res = max(res,1+dist(t))
    dp[s] = res
    return res

ans = 0
for i in range(N):
    ans = max(ans,dist(i))

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import defaultdict,deque
INF = float('inf')
N,M = map(int,input().split())

d_from = defaultdict(lambda:[])
d_to = defaultdict(lambda:[])

for _ in range(M):
    x,y = map(int,input().split())
    x -= 1
    y -= 1
    d_from[x].append(y)
    d_to[y].append(x)

dp = [-INF]*N
for i in range(N):
    if d_from[i] == []:
        dp[i] = 0
        que = deque([])
        for j in d_to[i]:
            dp[j] = max(dp[i]+1,dp[j])
            que.append(j)
        while que:
            j = que.pop()
            for k in d_to[j]:
                dp[k] = max(dp[j]+1,dp[k])
                que.append(k)
print(max(dp))

Fixed submission:
from collections import defaultdict
import sys
sys.setrecursionlimit(10**9)
N,M = map(int,input().split())

d = defaultdict(lambda:[])

for _ in range(M):
    x,y = map(int,input().split())
    x -= 1
    y -= 1
    d[x].append(y)

dp = [-1]*N

def dist(s):
    if dp[s] >= 0:
        return dp[s]
    if d[s] == []:
        dp[s] = 0
        return 0
    res = 0
    for t in d[s]:
        res = max(res,1+dist(t))
    dp[s] = res
    return res

ans = 0
for i in range(N):
    ans = max(ans,dist(i))

print(ans)