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)

x, y = map(int, input().split())

if (x+y)%3 != 0:
    print(0)
    exit(0)

n = (2*y-x)//3
m = (2*x-y)//3

if n<0 or m<0:
    print(0)
    exit(0)

mi = min(n,m)
num = n+m
mod = 10**9+7
a = 1
b = 1

for i in range(mi):
    a = a*(num-i)%mod
    b = b*(mi-i)%mod

print((a*pow(b,mod-2,mod))%mod)