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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<bits/stdc++.h>
#include<math.h>
using namespace std;
int main(){
int X,Y;
cin >> X >> Y;
if((X+Y)%3!=0)
cout << 0 << endl;
else if( Y > X*2|| X > Y*2)
cout << 0 << endl;
else{
int n = (X+Y)/3;
int r = min(X,Y)-n;
    long ans = 1;
    for (int i = n; i > n - r; --i) {
        ans = ans*i;
    }
    for (int i = 1; i < r + 1 ; ++i) {
        ans = ans / i;
    }
    cout << ans % 1000000007 << endl; 
}


}