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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

/*
String[]の初期化、整数の型がlongになっているか
関数（splitとかcontainsとか）内では"+"とかは正規表現となされるので"\\+"とする
'+'とかは問題ない
配列のコピーはarr1=arr.clone()
HashSet<>[]はおのおの初期化した？？？？？
負の数の割り算は絶対値が小さいほうに切り捨てられるex.-1/2=0ので、ある値>kみたいな式だとバグらせやすいので注意
ある値>=kとして、切り上げた値をとるべき
toLowerCase()はアルファベット以外に対して行ってもエラーにはならない
ArrayDequeではpopとpushはStackと同じ操作になる
//import static java.lang.System.*; はしばらく使わなそう(使うとしてもexit(0)のためだけ)←return;でよさそう
最後にout.flush()またはout.close()をするのを忘れずに！
Point2D p2d=new Point2D.Double(d1,d2);
Line2D l2d=new Line2D.Double(x1,y1,x2,y2);
Line2D l2d_0=new Line2D.Double(x11,y11,x22,y22);
if(l2d.intersectsLine(l2d_0))
next_Permutationを使うときはソート済み配列かどうか確認！！
参照型の比較するときはプリミティブ型にキャストするか.compare,.equalsを使う！
ex.map.get(0)=100,map.get(1)=100でもmap.get(0)==map.get(1)がfalseになりうる
*/
//入力終了→Ctrl+D
import java.io.*;
import java.awt.*;
import java.util.*;
import java.awt.geom.*;
import static java.lang.Math.*;

public class Main {
    public static void main(String[] args) {
        COM.Init(1000000);
        int x=sc.nextInt();
        int y=sc.nextInt();
        long ans=0;
        int k=0,l=0;
        int k3=2*y-x;
        int l3=2*x-y;
        if (k3>=0&&l3>=0&&k3%3==0&&l3%3==0){
            k=k3/3;
            l=l3/3;
            ans=COM.C(k+l,l);
        }
        out.println(ans);
        out.close();
    }

    static long mod = 1000000007;

    static boolean nextPermutation(int[] a) {
        int t = a.length - 1;
        int tt = t + 1;
        for (int i = t; i > 0; --i) {
            if (a[i - 1] < a[i]) {
                int swapIndex = Find(a[i - 1], a, i, t);
                int temp = a[swapIndex];
                a[swapIndex] = a[i - 1];
                a[i - 1] = temp;
                Arrays.sort(a, i, tt);
                return true;
            }
        }
        return false;
    }

    static int Find(int dest, int[] a, int s, int e) {
        if (s == e) {
            return s;
        }
        int m = (s + e + 1) / 2;
        return a[m] <= dest ? Find(dest, a, s, m - 1) : Find(dest, a, m, e);
    }

    static long gcd(long a, long b) {
        return b == 0 ? a : gcd(b, a % b);
    }

    static long power(long x, long n) {
        if (n == 0) {
            return 1;
        } else if (n % 2 == 0) {
            long e = power(x, n / 2) % mod;
            return (e * e) % mod;
        } else {
            return (x * power(x, n - 1)) % mod;
        }
    }

    //クラスカル法(0-indexed) n頂点m辺
    static HashSet<Integer> Kruskal(int[] u, int[] v, long[] cost, int n) {
        int m = u.length;
        UF.Init(n);
        PriorityQueue<Integer> S = new PriorityQueue<>((p1, p2) -> Long.compare(cost[p1], cost[p2]));
        for (int i = 0; i < m; i++) {
            S.add(i);
        }
        //最小全域木をなす辺
        HashSet<Integer> F = new HashSet<>();
        while (!S.isEmpty()) {
            int e = S.poll();
            if (!UF.same(u[e], v[e])) {
                F.add(e);
                UF.unite(u[e], v[e]);
            }
        }
        return F;
    }

    //ワーシャルフロイド(0-indexed)
    static void WF(double[][] d, int kappa, int lambda) {
        int n = kappa + lambda;
        for (int k = 0; k < n; k++) {
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < n; j++) {
                    if (d[i][k] != -1 && d[k][j] != -1) {
                        d[i][j] = d[i][j] > d[i][k] + d[k][j] ? d[i][k] + d[k][j] : d[i][j];
                    }
                }
            }
        }
    }

    //Union Find木(0-indexed)
    static class UF {
        static int[] d;

        static void Init(int size) {
            d = new int[size];
            for (int i = 0; i < size; i++) {
                d[i] = -1;
            }
        }

        static boolean unite(int x, int y) {
            x = find(x);
            y = find(y);
            if (x == y) return false;
            if (d[x] > d[y]) {
                int t = d[y];
                d[y] = d[x];
                d[x] = t;
            }
            d[x] += d[y];
            d[y] = x;
            return true;
        }

        static boolean same(int x, int y) {
            return find(x) == find(y);
        }

        static int find(int k) {
            return d[k] < 0 ? k : (d[k] = find(d[k]));
        }

        static int size(int k) {
            return -d[find(k)];
        }
    }

    //Combination
    static class COM {
        static long[] fact;
        static long[] inv;

        static void Init(int n) {
            fact = new long[n];
            inv = new long[n];
            fact[0] = inv[0] = 1;
            for (int i = 1; i < n; i++) {
                fact[i] = (fact[i - 1]%mod * i) % mod;
                inv[i] = power(fact[i]%mod, mod - 2);
            }
        }

        static long C(int n, int k) {
            if (k > n) return 0;
            return ((fact[n] * inv[n - k]) % mod * inv[k]) % mod;
        }
    }

    static PrintWriter out = new PrintWriter(System.out);

    static class sc {
        static Scanner scanner = new Scanner(System.in);

        static String next() {
            return scanner.next();
        }

        static String nextLine() {
            return scanner.nextLine();
        }

        static int nextInt() {
            return Integer.parseInt(scanner.next());
        }

        static long nextLong() {
            return Long.parseLong(scanner.next());
        }

        static double nextDouble() {
            return Double.parseDouble(scanner.next());
        }

        static boolean hasNext() {
            return scanner.hasNext();
        }

        static boolean hasNextInt() {
            return scanner.hasNextInt();
        }

        static boolean hasNext(String str) {
            return scanner.hasNext(str);
        }
    }

}