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: Time Limit Exceeded)

# -*- coding: utf-8 -*-
import sys
X, Y = map(int, input().split())

mod = 10 ** 9 + 7

a = (2 * Y - X) / 3
b = (2 * X - Y) / 3
if a < 0 or b < 0 or (X + Y) % 3 != 0:
    print(0)
    sys.exit()

a = int(a)
b = int(b)
N = a + b
f = [1]
invf = [1]
for i in range(1,a+b+1):
    f.append((f[-1] * i) % mod)
    invf.append(pow(f[i], mod-2, mod))
ans = f[a+b] * (invf[a] * invf[b] % mod) % mod
print(ans)