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: Accepted)

#ABC145D

MOD = 10 ** 9 + 7
import math
x,y = map(int,input().split())
a,b = -1,-1

for i in range(x+1):
    m = 0
    if (x-i) % 2 == 0:
        m = (x-i) // 2
        if 2*i + m == y:
            if i >= 0 and m >= 0:
                a = i
                b = m
                break
    else:
        continue
        
def kai(x):
    an = 1
    for i in range(1,x+1):
        an = an * i % MOD
        
    return an

def pow_k(x, n):
    if n == 0:
        return 1

    K = 1
    while n > 1:
        if n % 2 != 0:
            K = K * x % MOD
        x = x ** 2 % MOD
        n //= 2

    return K * x % MOD

al = kai(a)
be = kai(b)

if a == -1 and b == -1:
    print(0)
else:
    print(( kai(a+b) * pow_k(al,MOD-2)) % MOD * pow_k(be,MOD-2) % MOD )