Buggy python submission (verdict: Runtime Error):
from functools import reduce
from fractions import gcd
import math
import bisect
import itertools
import sys
sys.setrecursionlimit(10**7)
input = sys.stdin.readline
INF = float("inf")
MOD = 1000000007


def bellman_ford(edges, num_v, sv):
    #グラフの初期化
    dist = [INF] * num_v
    dist[sv]=0
    
    #辺の緩和
    for i in range(num_v):
        for edge in edges:
            s, t, d = edge[0], edge[1], edge[2]
            if dist[s] != INF and dist[t] > dist[s] + d:
                dist[t] = dist[s] + d
                if i == num_v - 1:
                    return -1       # 負の閉路が存在する
    
    return dist


def main():
    N, M = map(int, input().split())

    edges = []
    for _ in range(M):
        a, b, c = map(int, input().split())
        a -= 1; b -= 1; c *= -1
        edges.append([a, b, c])
    
    dist = bellman_ford(edges, N, 0)

    if dist == 0:
        print("inf")
    else:
        print(-dist[N-1])


if __name__ == '__main__':
    main()


Fixed submission:
N, M = map(int, input().split())
a = [0]*M
b = [0]*M
c = [0]*M
for i in range(M):
	x, y, z = map(int, input().split())
	a[i] = x
	b[i] = y
	c[i] = -z

INF = float('inf')
d = [INF]*N
d[0] = 0

for _ in range(N-1):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]

ans = -d[N-1]

negative = [False]*N
for _ in range(N):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]
			negative[b[i]-1] = True
		if negative[a[i]-1]==True:
			negative[b[i]-1] = True

if negative[N-1]==True:
	print('inf')
else:
	print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
N, M = map(int, input().split())
a = [0]*M
b = [0]*M
c = [0]*M
for i in range(M):
	x, y, z = map(int, input().split())
	a[i] = x
	b[i] = y
	c[i] = -z

INF = float('inf')
d = [INF]*N
d[0] = 0

for _ in range(N-1):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]

ans = -d[N-1]

negative = [False]*N
for _ in range(N):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]
			negative[b[i]-1] = True
		if negative[a[i]-1]==True:
			negative[b[i]-1] = True

if negative[N-1]==True:
	print('inf')
else:
	print(ans)

Fixed submission:
N, M = map(int, input().split())
a = [0]*M
b = [0]*M
c = [0]*M
for i in range(M):
	x, y, z = map(int, input().split())
	a[i] = x
	b[i] = y
	c[i] = -z

INF = float('inf')
d = [INF]*N
d[0] = 0

for _ in range(N-1):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]

ans = -d[N-1]

negative = [False]*N
for _ in range(N):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]
			negative[b[i]-1] = True
		if negative[a[i]-1]==True:
			negative[b[i]-1] = True

if negative[N-1]==True:
	print('inf')
else:
	print(ans)

Buggy python submission (verdict: Runtime Error):
N, M = map(int, input().split())
a = [0]*M
b = [0]*M
c = [0]*M
for i in range(M):
	x, y, z = map(int, input().split())
	a[i] = x
	b[i] = y
	c[i] = -z

INF = float('inf')
d = [INF]*N
d[0] = 0

for _ in range(N-1):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]

ans = -d[N-1]

negative = [False]*N
for _ in range(N):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]
			negative[i] = True
		if negative[a[i]-1]==True:
			negative[b[i]-1] = True

if negative[N-1]==True:
	print('inf')
else:
	print(ans)

Fixed submission:
N, M = map(int, input().split())
a = [0]*M
b = [0]*M
c = [0]*M
for i in range(M):
	x, y, z = map(int, input().split())
	a[i] = x
	b[i] = y
	c[i] = -z

INF = float('inf')
d = [INF]*N
d[0] = 0

for _ in range(N-1):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]

ans = -d[N-1]

negative = [False]*N
for _ in range(N):
	for i in range(M):
		if d[a[i]-1]==INF:
			continue
		if d[b[i]-1]>d[a[i]-1]+c[i]:
			d[b[i]-1] = d[a[i]-1] + c[i]
			negative[b[i]-1] = True
		if negative[a[i]-1]==True:
			negative[b[i]-1] = True

if negative[N-1]==True:
	print('inf')
else:
	print(ans)