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: Wrong Answer)

X, Y = map(int,input().split())
X, Y = (Y, X) if X < Y else (X, Y)

MOD = 10**9 + 7

def comb(n, r):
    n += 1
    over = 1
    under = 1
    for i in range(1,r + 1):
      over = over * (n - i) % MOD
      under = under * i % MOD
    return over * pow(under,MOD - 2,MOD) % MOD

if (X + Y) % 3 != 0:
    print(0)
else:
    diff = Y - X
    X -= diff
    one = X // 3 + diff
    two = X // 3
    print(comb(one+two, one))