Buggy python submission (verdict: Runtime Error):
"""
Bellman-Ford
始点と終点が決められているとき、
経由する閉路のみを確認する必要がある。
"""
def Bellman_Ford2(g,start,goal):
    n = len(g)    
    dist = [float("inf")]*n
    dist[start] = 0
    a0 = float("inf")
    for i in range(2*n):
        updated = False
        for v in range(n):
            for to, cost in g[v]:
                if dist[to] <= dist[v] + cost: continue
                if i < n:
                    dist[to] = dist[v] + cost
                else:
                    dist[to] = -float("inf")
                updated = True
        if not updated: break
    if dist[goal] == -float("inf"): return None, False
#    if negative[-1]: return None, False
    return dist, True
 

import sys
sys.setrecursionlimit(10**6)
readline = sys.stdin.readline #文字列入力のときは注意

n,m,p = [int(i) for i in readline().split()]
g = [[] for _ in range(n)]
for i in range(m):
    a,b,c = [int(i) for i in readline().split()]
    g[a-1].append((b-1,-c))
#    g[b-1].append((a-1,c))

dist, TF = Bellman_Ford2(g,0,-1)

if TF: print(-dist[-1])
else: print("inf")


Fixed submission:
# coding: utf-8
# Your code here!

def Bellman_Ford(g,start):
    n = len(g)    
    dist = [float("inf")]*n
    dist[start] = 0
    a0 = float("inf")
    negative = [False]*n
    for i in range((2*n)):
        updated = False
        for v in range(n):
            for to, cost in g[v]:
                if negative[v]: negative[to] = True
                if dist[to] <= dist[v] + cost: continue
                dist[to] = dist[v] + cost
                updated = True
                if i >= n:
                    negative[to] = True
        if not updated: break
    if negative[-1]: return None, False
    return dist, True


import sys
sys.setrecursionlimit(10**6)
readline = sys.stdin.readline #文字列入力のときは注意

n,m = [int(i) for i in readline().split()]
g = [[] for _ in range(n)]
for i in range(m):
    a,b,c = [int(i) for i in readline().split()]
    g[a-1].append((b-1,-c))
#    g[b-1].append((a-1,c))

dist, TF = Bellman_Ford(g, 0)

if TF: print(-dist[-1])
else: print("inf")

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

def Bellman_Ford(g,start):
    n = len(g)    
    dist = [float("inf")]*n
    dist[start] = 0
    a0 = float("inf")
    
    for i in range(2):
        updated = False
        for v in range(n):
            for to, cost in g[v]:
                if dist[to] <= dist[v] + cost: continue
                dist[to] = dist[v] + cost
                updated = True
        if not updated: break
        if i <= 0: a0 = dist[-1]
    if a0 != dist[-1]: return None, False
    return dist, True


import sys
sys.setrecursionlimit(10**6)
readline = sys.stdin.readline #文字列入力のときは注意

n,m = [int(i) for i in readline().split()]
g = [[] for _ in range(n)]
for i in range(m):
    a,b,c = [int(i) for i in readline().split()]
    g[a-1].append((b-1,-c))
#    g[b-1].append((a-1,c))

dist, TF = Bellman_Ford(g, 0)

if TF: print(-dist[-1])
else: print("inf")

if n==2 and m==2: assert False



Fixed submission:
# coding: utf-8
# Your code here!

def Bellman_Ford(g,start):
    n = len(g)    
    dist = [float("inf")]*n
    dist[start] = 0
    a0 = float("inf")
    negative = [False]*n
    for i in range((2*n)):
        updated = False
        for v in range(n):
            for to, cost in g[v]:
                if negative[v]: negative[to] = True
                if dist[to] <= dist[v] + cost: continue
                dist[to] = dist[v] + cost
                updated = True
                if i >= n:
                    negative[to] = True
        if not updated: break
    if negative[-1]: return None, False
    return dist, True


import sys
sys.setrecursionlimit(10**6)
readline = sys.stdin.readline #文字列入力のときは注意

n,m = [int(i) for i in readline().split()]
g = [[] for _ in range(n)]
for i in range(m):
    a,b,c = [int(i) for i in readline().split()]
    g[a-1].append((b-1,-c))
#    g[b-1].append((a-1,c))

dist, TF = Bellman_Ford(g, 0)

if TF: print(-dist[-1])
else: print("inf")