Buggy python submission (verdict: Time Limit Exceeded):
mod=10**9+7
fact=[1]
for i in range(1,5*10**5+1):
  fact.append((fact[-1]*i)%mod)
revfact=[1]
for i in range(1,5*10**5+1):
  revfact.append(pow(fact[i],mod-2,mod))
n,m=map(int,input().split())
patternA=(fact[m]*revfact[n]*revfact[m-n]*fact[n])%mod
patternB=0
for i in range(n+1):
  tmp=(fact[n]*revfact[i]*revfact[n-i])*(fact[m-i]*revfact[n-i]*revfact[m-n])*fact[n-i]
  if i%2==0:
    patternB+=tmp
    patternB%=mod
  else:
    patternB-=tmp
    patternB%=mod
print((patternA*patternB)%mod)

Fixed submission:
mod=10**9+7
fact=[1]
for i in range(1,5*10**5+1):
  fact.append((fact[-1]*i)%mod)
revfact=[1]
for i in range(1,5*10**5+1):
  revfact.append(pow(fact[i],mod-2,mod))
n,m=map(int,input().split())
patternA=(fact[m]*revfact[n]*revfact[m-n]*fact[n])%mod
patternB=patternA
for i in range(1,n+1):
  tmp=(fact[n]*revfact[i]*revfact[n-i])*(fact[m-i]*revfact[n-i]*revfact[m-n])*fact[n-i]
  if i%2==0:
    patternB+=tmp
    patternB%=mod
  else:
    patternB-=tmp
    patternB%=mod
print((patternA*patternB)%mod)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
input=sys.stdin.readline

def calc():
  mod=10**9+7
  fact=[1]
  for i in range(1,5*10**5+1):
    fact.append((fact[-1]*i)%mod)
  revfact=[1]
  for i in range(1,5*10**5+1):
    revfact.append(pow(fact[i],mod-2,mod))
  n,m=map(int,input().split())
  patternA=(fact[m]*revfact[n]*revfact[m-n]*fact[n])%mod
  patternB=patternA
  for i in range(1,n+1):
    tmp=(fact[n]*revfact[i]*revfact[n-i])*(fact[m-i]*revfact[n-i]*revfact[m-n])*fact[n-i]
    if i%2==0:
      patternB+=tmp
      patternB%=mod
    else:
      patternB-=tmp
      patternB%=mod
  print((patternA*patternB)%mod)
calc()

Fixed submission:
mod=10**9+7
fact=[1]
for i in range(1,5*10**5+1):
  fact.append((fact[-1]*i)%mod)
revfact=[1]
for i in range(1,5*10**5+1):
  revfact.append(pow(fact[i],mod-2,mod))
n,m=map(int,input().split())
patternA=(fact[m]*revfact[n]*revfact[m-n]*fact[n])%mod
patternB=patternA
for i in range(1,n+1):
  tmp=(fact[n]*revfact[i]*revfact[n-i])*(fact[m-i]*revfact[n-i]*revfact[m-n])*fact[n-i]
  if i%2==0:
    patternB+=tmp
    patternB%=mod
  else:
    patternB-=tmp
    patternB%=mod
print((patternA*patternB)%mod)

Buggy python submission (verdict: Time Limit Exceeded):
mod=10**9+7
fact=[1]
for i in range(1,5*10**5+1):
  fact.append((fact[-1]*i)%mod)
revfact=[1]
for i in range(1,5*10**5+1):
  revfact.append(pow(fact[i],mod-2,mod))
n,m=map(int,input().split())
patternA=(fact[m]*revfact[n]*revfact[m-n]*fact[n])%mod
patternB=patternA
for i in range(1,n+1):
  tmp=(fact[n]*revfact[i]*revfact[n-i])*(fact[m-i]*revfact[n-i]*revfact[m-n])*fact[n-i]
  if i%2==0:
    patternB+=tmp
    patternB%=mod
  else:
    patternB-=tmp
    patternB%=mod
print((patternA*patternB)%mod)


Fixed submission:
mod=10**9+7
fact=[1]
for i in range(1,5*10**5+1):
  fact.append((fact[-1]*i)%mod)
revfact=[1]
for i in range(1,5*10**5+1):
  revfact.append(pow(fact[i],mod-2,mod))
n,m=map(int,input().split())
patternA=(fact[m]*revfact[n]*revfact[m-n]*fact[n])%mod
patternB=patternA
for i in range(1,n+1):
  tmp=(fact[n]*revfact[i]*revfact[n-i])*(fact[m-i]*revfact[n-i]*revfact[m-n])*fact[n-i]
  if i%2==0:
    patternB+=tmp
    patternB%=mod
  else:
    patternB-=tmp
    patternB%=mod
print((patternA*patternB)%mod)