Buggy python submission (verdict: Runtime Error):
# coding: utf-8
# Your code here!
from functools import lru_cache


n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

#memo = [-1]*n

@lru_cache(maxsize=None)
def longest(v): #nを終点とするlongest
#    if memo[v] >= 0:
#        return memo[v]
#    else:
    if oya[v]:
        return max([longest(j)+1 for j in oya[v]])
    else:
        return 0
        
    

memo = [longest(i) for i in range(n)]

print(max(memo))    
#print(memo)


Fixed submission:
# coding: utf-8
# Your code here!
import sys
sys.setrecursionlimit(10**6)

n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

memo = [-1]*n

def longest(v): #nを終点とするlongest
    if memo[v] >= 0:
        return memo[v]
    else:
        c=0
        for j in oya[v]:
            c = max(c,longest(j)+1)
        memo[v] = c
        return c

for i in range(n):
    longest(i)

print(max(memo))    
#print(memo)

Buggy python submission (verdict: Time Limit Exceeded):
# coding: utf-8
# Your code here!
from functools import lru_cache
import sys
sys.setrecursionlimit(10**6)

n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

#memo = [-1]*n

@lru_cache(maxsize=None)
def longest(v): #nを終点とするlongest
#    if memo[v] >= 0:
#        return memo[v]
#    else:
    c=0
    for j in oya[v]:
        c = max(c,longest(j)+1)
#    memo[v] = c
    return c

memo = [longest(i) for i in range(n)]

print(max(memo))    
#print(memo)


Fixed submission:
# coding: utf-8
# Your code here!
import sys
sys.setrecursionlimit(10**6)

n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

memo = [-1]*n

def longest(v): #nを終点とするlongest
    if memo[v] >= 0:
        return memo[v]
    else:
        c=0
        for j in oya[v]:
            c = max(c,longest(j)+1)
        memo[v] = c
        return c

for i in range(n):
    longest(i)

print(max(memo))    
#print(memo)

Buggy python submission (verdict: Time Limit Exceeded):
# coding: utf-8
# Your code here!
from functools import lru_cache
import sys
sys.setrecursionlimit(10**6)

n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

#memo = [-1]*n

@lru_cache()
def longest(v): #nを終点とするlongest
#    if memo[v] >= 0:
#        return memo[v]
#    else:
    c=0
    for j in oya[v]:
        c = max(c,longest(j)+1)
#    memo[v] = c
    return c

memo = [longest(i) for i in range(n)]

print(max(memo))    
#print(memo)


Fixed submission:
# coding: utf-8
# Your code here!
import sys
sys.setrecursionlimit(10**6)

n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

memo = [-1]*n

def longest(v): #nを終点とするlongest
    if memo[v] >= 0:
        return memo[v]
    else:
        c=0
        for j in oya[v]:
            c = max(c,longest(j)+1)
        memo[v] = c
        return c

for i in range(n):
    longest(i)

print(max(memo))    
#print(memo)

Buggy python submission (verdict: Runtime Error):
# coding: utf-8
# Your code here!
from functools import lru_cache


n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

#memo = [-1]*n

@lru_cache()
def longest(v): #nを終点とするlongest
#    if memo[v] >= 0:
#        return memo[v]
#    else:
    c=0
    for j in oya[v]:
        c = max(c,longest(j)+1)
#    memo[v] = c
    return c

memo = [longest(i) for i in range(n)]

print(max(memo))    
#print(memo)


Fixed submission:
# coding: utf-8
# Your code here!
import sys
sys.setrecursionlimit(10**6)

n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

memo = [-1]*n

def longest(v): #nを終点とするlongest
    if memo[v] >= 0:
        return memo[v]
    else:
        c=0
        for j in oya[v]:
            c = max(c,longest(j)+1)
        memo[v] = c
        return c

for i in range(n):
    longest(i)

print(max(memo))    
#print(memo)

Buggy python submission (verdict: Runtime Error):
# coding: utf-8
# Your code here!

n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

memo = [-1]*n

def longest(v): #nを終点とするlongest
    if memo[v] >= 0:
        return memo[v]
    else:
        c=0
        for j in oya[v]:
            c = max(c,longest(j)+1)
        memo[v] = c
        return c

for i in range(n):
    longest(i)

print(max(memo))    
#print(memo)


Fixed submission:
# coding: utf-8
# Your code here!
import sys
sys.setrecursionlimit(10**6)

n,m = [int(i) for i in input().split()]

#ko = [[] for _ in range(n)]
oya = [[] for _ in range(n)]
for _ in range(m):
    x,y = [int(i)-1 for i in input().split()]
#    ko[x].append(y)
    oya[y].append(x)

memo = [-1]*n

def longest(v): #nを終点とするlongest
    if memo[v] >= 0:
        return memo[v]
    else:
        c=0
        for j in oya[v]:
            c = max(c,longest(j)+1)
        memo[v] = c
        return c

for i in range(n):
    longest(i)

print(max(memo))    
#print(memo)