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: Time Limit Exceeded)

def comb(n, k, p):
    from math import factorial
    if n < 0 or k < 0 or n < k: return 0
    if n == 0 or k == 0: return 1
    a = factorial(n) % p
    b = factorial(k) % p
    c = factorial(n - k) % p
    return (a * power_func(b, p - 2, p) * power_func(c, p - 2, p)) % p


def power_func(a, b, p):
    if b == 0: return 1
    if b % 2 == 0:
        d = power_func(a, b // 2, p)
        return d * d % p
    if b % 2 == 1:
        return (a * power_func(a, b - 1, p)) % p

X, Y = map(int, input().split())
ans = 0
mod = 10 ** 9 + 7
if X > Y:
    X, Y = Y, X
##Yのほうが大きいとして考える
if not (X + Y) % 3 == 0 or Y > 2 * X:
    ans = 0
else:
    n = (X + Y) // 3
    d = Y - X
    a = 2*n - X
    ans = comb(n, a, mod)
print(ans)