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: Accepted)

#include <limits.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <algorithm>
#include <cassert>
#include <cfloat>
#include <complex>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> P;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef deque<ll> dqll;

vector<ll> cst;
vector<vector<P>> lst;

const ll mod = 1000000007;
ll fact[10200200];
ll invfact[10200200];

inline ll take_mod(ll a) { return (a % mod + mod) % mod; }

inline ll add(ll a, ll b) { return take_mod(a + b); }

inline ll sub(ll a, ll b) { return take_mod(a - b); }

inline ll mul(ll a, ll b) { return take_mod(a * b); }

inline ll pow(ll x, ll n) {
  ll res = 1LL;
  while (n > 0) {
    if (n & 1) res = mul(res, x);
    x = mul(x, x);
    n >>= 1;
  }
  return res;
}

ll mod_inv(ll x) { return pow(x, mod - 2); }

void make_fact(ll n) {
  fact[0] = 1;
  ll res = 1;
  for (int i = 1; i <= n; i++) {
    fact[i] = res;
    res = mul(res, i + 1);
  }
}

void make_invfact(ll n) {
  invfact[0] = 1;
  invfact[n] = mod_inv(fact[n]);
  for (int i = n - 1; i >= 1; i--) {
    invfact[i] = mul(invfact[i + 1], i + 1);
  }
}

ll perm(ll n, ll k) { return mul(fact[n], invfact[n - k]); }

ll comb(ll n, ll k) { return mul(mul(fact[n], invfact[n - k]), invfact[k]); }

int main() {
  ll X, Y;
  cin >> X >> Y;

  if ((X + Y) % 3 != 0) {
    cout << 0 << endl;
    return 0;
  }

  ll dai, sho;
  dai = max(X, Y);
  sho = min(X, Y);

  if(dai > 2*sho){
    cout << 0 << endl;
    return 0;
  }

  ll mNum = (dai + sho) / 3;

  ll kk = 1;
  for (ll i = 0; i <= mNum; i++) {
    if(i + 2*(mNum - i) == sho){
      kk = i;
      break;
    }
  } // i

  make_fact(10000000);
  make_invfact(10000000);

  ll ans = comb(mNum, kk);

  cout << ans << endl;

  return 0;
}