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 C (GCC 5.4.1) (judge verdict: Runtime Error)

#include <stdio.h>
long int factorial(long int num) {
  long int sol = 1;
  for (long int n = 0; n < num; n++) {
    sol = sol * (n + 1);
  }
  return sol;
}

long int main(void) {
  long int x, y;
  long int r = scanf("%ld %ld", &x, &y);
  long int move = (long int)(x + y) / 3;
  long int abs = y - x;
  printf("%ld", (factorial(move) /
                       (factorial((move - abs) / 2) *
                        factorial((move - abs) / 2 + abs)) %
                       1000000007));
}