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)

def nCr(n,r,mod = 10**9+7):
    r = min(n-r,r)
    numer = denom = 1
    for i in range(1,r+1):
        numer = numer * (n+1-i) %mod
        denom = denom * i % mod
    return numer * pow(denom,mod-2,mod) %mod
    
x,y = map(int,input().split())
if (x+y) %3 != 0 or not (x <= 2*y and y <= 2*x ):
    print(0)
    exit()
    
a = (x + y) //3
n = y - a

print(nCr(a,n))