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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php
define("MOD", 10**9+7);

list($x, $y) = ints();
$diff = abs($x-$y);
$st = ($x + $y) - $diff * 3;
if($st < 0 || ($x+$y) % 3 != 0){
  echo 0;exit;
}
$a = $st/6;
$b = $st/6+$diff;
echo nCm($a+$b, $a);

function str(){
  return trim(fgets(STDIN));
}
function ints(){
  return array_map("intval", explode(" ", trim(fgets(STDIN))));
}
function int(){
  return intval(trim(fgets(STDIN)));
}
function o(...$val){
    if(count($val)==1)$val = array_shift($val);
    $trace = debug_backtrace();
    echo $trace[0]['line'].")";
    if(is_array($val)){
        if(count($val) == 0){
            echo "empty array";
        }elseif(!is_array(current($val))){
            echo "array:";
            echo implode(" ", $val)."\n";
        }else{
            echo "array:array\n";
            foreach($val as $row){
                echo implode(" ", $row)."\n";
            }
        }
    }else{
        echo $val."\n";
    }
}

//足し算
function addm($a, $b){
    return ($a + $b) % MOD;
}
//引き算
function subm($a, $b){
    return ($a + MOD - $b) % MOD;
}
//掛け算
function mulm($a, $b){
    return ($a * $b) % MOD;
}
//割り算
function divm($a, $b){
    return mulm($a, powerm($b, MOD - 2));
}
//累乗
function powerm($a, $b){
    if($b == 0)return 1;
    if($b & 1)return (powerm($a, $b>>1)**2 % MOD) * $a % MOD;
    return powerm($a, $b>>1)**2 % MOD;
}
//階乗
function factorialm($a){
    if($a == 0)return 1;
    return factorialm($a - 1) * $a % MOD;
}
//順列
function nPm($n, $m){
    if($m == 0)return 1;
    return nPm($n, $m - 1) * ($n - $m + 1) % MOD;
}
//組み合わせ
function nCm($n, $m){
    $m = min($m, $n-$m);
    return divm(nPm($n, $m), factorialm($m));
}