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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

func getScanner(fp *os.File) *bufio.Scanner {
	scanner := bufio.NewScanner(fp)
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 500001), 500000)
	return scanner
}

func getNextString(scanner *bufio.Scanner) string {
	scanner.Scan()
	return scanner.Text()
}

func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}

func getNextInt64(scanner *bufio.Scanner) int64 {
	i, _ := strconv.ParseInt(getNextString(scanner), 10, 64)
	return i
}

func getNextUint64(scanner *bufio.Scanner) uint64 {
	i, _ := strconv.ParseUint(getNextString(scanner), 10, 64)
	return i
}

func getNextFloat64(scanner *bufio.Scanner) float64 {
	i, _ := strconv.ParseFloat(getNextString(scanner), 64)
	return i
}

func main() {
	fp := os.Stdin
	wfp := os.Stdout

	if os.Getenv("MASPY") == "ますピ" {
		fp, _ = os.Open(os.Getenv("BEET_THE_HARMONY_OF_PERFECT"))
	}
	if os.Getenv("MASPYPY") == "ますピッピ" {
		wfp, _ = os.Create(os.Getenv("NGTKANA_IS_GENIUS10"))
	}

	scanner := getScanner(fp)
	writer := bufio.NewWriter(wfp)

	x := getNextInt(scanner)
	y := getNextInt(scanner)

	if x > y {
		x, y = y, x
	}
	if x*2 < y || (x+y)%3 != 0 {
		fmt.Println(0)
		return
	}

	c := (x + y) / 3
	fac := make([]int64, c+1)
	fac[0] = 1
	mod := int64(1e9 + 7)
	for i := 0; i < c; i++ {
		fac[i+1] = (fac[i] * int64(i+1)) % mod
	}

	fmt.Fprintln(writer, ((fac[c]*inv(fac[x-c])%mod)*inv(fac[2*c-x]))%mod)
	writer.Flush()
}

func inv(n int64) int64 {
	var ii [32]int64
	mod := int64(1e9 + 7)
	ii[0] = n
	for i := 1; i < 31; i++ {
		ii[i] = (ii[i-1] * ii[i-1]) % mod
	}
	var ans int64
	ans = 1
	m := mod - 2
	for i := 30; i >= 0; i-- {
		for m >= 1<<uint(i) {
			ans = (ans * ii[i]) % mod
			m -= 1 << uint(i)
		}
	}

	return ans
}