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 Python (3.4.3) (judge verdict: Accepted)

# ABC145 D Knight
def factorial_mod(n, mod):
    a = 1
    for i in range(1, n + 1):
        a *= i
        a %= mod
    return a

def comb_mod(n, k, mod):
    if k > n or k < 0:
        return 0
    fact_n = factorial_mod(n, mod)
    fact_k = factorial_mod(k, mod)
    fact_n_k = factorial_mod(n - k, mod)
    return (fact_n * pow(fact_k, mod - 2, mod) * pow(fact_n_k, mod - 2, mod)) % mod

x, y = map(int, input().split())
MOD = 10**9 + 7
if (2 * x - y) % 3 != 0 or (2 * y - x) % 3 != 0:
    print(0)
    exit()
i = (2 * x - y) // 3
j = (2 * y - x) // 3
print(comb_mod(i + j, i, MOD))