Problem:
Score : 400 points

Problem Statement

There is a knight - the chess piece - at the origin (0, 0) of a two-dimensional grid.

When the knight is at the square (i, j), it can be moved to either (i+1,j+2) or (i+2, j+1).

In how many ways can the knight reach the square (X, Y)?

Find the number of ways modulo 10^9 + 7.

Constraints

1 \leq X \leq 10^6

1 \leq Y \leq 10^6

All values in input are integers.

Input

Input is given from Standard Input in the following format:

X Y

Output

Print the number of ways for the knight to reach (X, Y) from (0, 0), modulo 10^9 + 7.

Sample Input 1

3 3

Sample Output 1

2

There are two ways: (0,0) \to (1,2) \to (3,3) and (0,0) \to (2,1) \to (3,3).

Sample Input 2

2 2

Sample Output 2

0

The knight cannot reach (2,2).

Sample Input 3

999999 999999

Sample Output 3

151840682

Print the number of ways modulo 10^9 + 7.

Solution in PyPy3 (2.4.0) (judge verdict: Wrong Answer)

# coding: utf-8
import re
import math
import itertools
from copy import deepcopy
import random
from heapq import heappop,heappush
import time
import os
import queue
import sys
import datetime
from functools import lru_cache
readline=sys.stdin.readline
sys.setrecursionlimit(2000)
#import numpy as np
alphabet="abcdefghijklmnopqrstuvwxyz"
mod=int(10**9+7)
inf=int(10**20)
week=["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"]
def yn(b):
    if b:
        print("yes")
    else:
        print("no")
def Yn(b):
    if b:
        print("Yes")
    else:
        print("No")
def YN(b):
    if b:
        print("YES")
    else:
        print("NO")
class union_find():
    def __init__(self,n):
        self.n=n
        self.P=[a for a in range(N)]
        self.rank=[0]*n
 
    def find(self,x):
        if(x!=self.P[x]):self.P[x]=self.find(self.P[x])
        return self.P[x]
 
    def same(self,x,y):
        return self.find(x)==self.find(y)
 
    def link(self,x,y):
        if self.rank[x]<self.rank[y]:
            self.P[x]=y
        elif self.rank[y]<self.rank[x]:
            self.P[y]=x
        else:
            self.P[x]=y
            self.rank[y]+=1
 
    def unite(self,x,y):
        self.link(self.find(x),self.find(y))
 
    def size(self):
        S=set()
        for a in range(self.n):
            S.add(self.find(a))
        return len(S)
def is_power(a,b):#aはbの累乗数か
    now=b
    while now<a:
        now*=b
    if now==a:return True
    else:return False
def bin_(num,size):
    A=[0]*size
    for a in range(size):
        if (num>>(size-a-1))&1==1:
            A[a]=1
        else:
            A[a]=0
    return A
def facs(n,mod_=0):
    A=[1]*(n+1)
    for a in range(2,len(A)):
        A[a]=A[a-1]*a
        if(mod_>0):A[a]%=mod_
    return A
def comb(n,r,mod_,fac):
    if(n-r<0):return 0
    return (fac[n]*pow(fac[n-r],mod_-2,mod_)*pow(fac[r],mod_-2,mod_))%mod_
def next_comb(num,size):
    x=num&(-num)
    y=num+x
    z=num&(~y)
    z//=x
    z=z>>1
    num=(y|z)
    if(num>=(1<<size)):return False
    else:
        return num
def get_primes(n,type="int"):
    A=[True]*(n+1)
    A[0]=False
    A[1]=False
    for a in range(2,n+1):
        if A[a]:
            for b in range(a*2,n+1,a):
                A[b]=False
    if(type=="bool"):return A
    B=[]
    for a in range(n+1):
        if(A[a]):B.append(a)
    return B
def is_prime(num):
    if(num<=2):return False
    i=2
    while i*i<=num:
       if(num%i==0):return False
       i+=1
    return True
def ifelse(a,b,c):
    if a:return b
    else:return  c
def join(A,c=" "):
    n=len(A)
    A=list(map(str,A))
    s=""
    for a in range(n):
        s+=A[a]
        if(a<n-1):s+=c
    return s
def factorize(n,type_="dict"):
    b = 2
    list_ = []
    while b * b <= n:
        while n % b == 0:
            n //= b
            list_.append(b)
        b+=1
    if n > 1:list_.append(n)
    if type_=="dict":
        dic={}
        for a in list_:
            if a in dic:
                dic[a]+=1
            else:
                dic[a]=1
        return dic
    elif type_=="list":
        return list_
    else:
        return None
def floor_(n,x=1):
    return x*(n//x)
def ceil_(n,x=1):
    return x*((n+x-1)//x)
#################################### ########
# coding: utf-8
import re
import math
import itertools
from copy import deepcopy
import random
from heapq import heappop,heappush
import time
import os
import queue
import sys
import datetime
from functools import lru_cache
readline=sys.stdin.readline
sys.setrecursionlimit(2000)
#import numpy as np
alphabet="abcdefghijklmnopqrstuvwxyz"
mod=int(10**9+7)
inf=int(10**20)
def yn(b):
    if b:
        print("yes")
    else:
        print("no")
def Yn(b):
    if b:
        print("Yes")
    else:
        print("No")
def YN(b):
    if b:
        print("YES")
    else:
        print("NO")
class union_find():
    def __init__(self,n):
        self.n=n
        self.P=[a for a in range(N)]
        self.rank=[0]*n
 
    def find(self,x):
        if(x!=self.P[x]):self.P[x]=self.find(self.P[x])
        return self.P[x]
 
    def same(self,x,y):
        return self.find(x)==self.find(y)
 
    def link(self,x,y):
        if self.rank[x]<self.rank[y]:
            self.P[x]=y
        elif self.rank[y]<self.rank[x]:
            self.P[y]=x
        else:
            self.P[x]=y
            self.rank[y]+=1
 
    def unite(self,x,y):
        self.link(self.find(x),self.find(y))
 
    def size(self):
        S=set()
        for a in range(self.n):
            S.add(self.find(a))
        return len(S)
def is_power(a,b):#aはbの累乗数か
    now=b
    while now<a:
        now*=b
    if now==a:return True
    else:return False
def bin_(num,size):
    A=[0]*size
    for a in range(size):
        if (num>>(size-a-1))&1==1:
            A[a]=1
        else:
            A[a]=0
    return A
def get_facs(n,mod_=0):
    A=[1]*(n+1)
    for a in range(2,len(A)):
        A[a]=A[a-1]*a
        if(mod>0):A[a]%=mod_
    return A
def comb(n,r,mod,fac):
    if(n-r<0):return 0
    return (fac[n]*pow(fac[n-r],mod-2,mod)*pow(fac[r],mod-2,mod))%mod
def next_comb(num,size):
    x=num&(-num)
    y=num+x
    z=num&(~y)
    z//=x
    z=z>>1
    num=(y|z)
    if(num>=(1<<size)):return False
    else:
        return num
def get_primes(n,type="int"):
    A=[True]*(n+1)
    A[0]=False
    A[1]=False
    for a in range(2,n+1):
        if A[a]:
            for b in range(a*2,n+1,a):
                A[b]=False
    if(type=="bool"):return A
    B=[]
    for a in range(n+1):
        if(A[a]):B.append(a)
    return B
def is_prime(num):
    if(num<=2):return False
    i=2
    while i*i<=num:
       if(num%i==0):return False
       i+=1
    return True
def ifelse(a,b,c):
    if a:return b
    else:return  c
def join(A,c=" "):
    n=len(A)
    A=list(map(str,A))
    s=""
    for a in range(n):
        s+=A[a]
        if(a<n-1):s+=c
    return s
def factorize(n,type_="dict"):
    b = 2
    list_ = []
    while b * b <= n:
        while n % b == 0:
            n //= b
            list_.append(b)
        b+=1
    if n > 1:list_.append(n)
    if type_=="dict":
        dic={}
        for a in list_:
            if a in dic:
                dic[a]+=1
            else:
                dic[a]=1
        return dic
    elif type_=="list":
        return list_
    else:
        return None
def floor_(n,x=1):
    return x*(n//x)
def ceil_(n,x=1):
    return x*((n+x-1)//x)
###################################################
def main():
    X,Y=map(int,input().split())
    if (X+Y)%3==0:
        n=(X+Y)//3
        X-=n
        Y-=n
        facs=get_facs(10**6+1,mod)
        if X<0 or Y<0:
            print(0)
            return
        ans=(facs[X+Y])*pow(facs[X]*facs[Y],mod-2,mod)
        print(ans)
    else:
        print(0)


main()