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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#include <math.h>
#define MAX(X, Y) (((X) > (Y))? (X) : (Y))
#define MIN(X, Y) (((X) < (Y))? (X) : (Y))
#define MOD 1000000007

// 大きな数の組み合わせを1000000007で割った余りを計算する方法
// https://www.smartbowwow.com/2018/10/mod1097.html
typedef long long ll;

// a^b mod p の計算
// 無理。iってなんなの。そこが不明。aには何を入れるのかな？
ll calc(ll a, ll b, ll p) {
    if (b == 0) {
        return 1;
    } else if (b % 2 == 0) {
        ll d = calc(a, b / 2, p);
        return (d * d) % p;
    } else {
        return (a * calc(a, b - 1, p)) % p;
    }
}

// https://qiita.com/drken/items/3b4fdf0a78e7a138cd9a
// 逆元を求めるアルゴリズム、及び mod mにおける割り算を実行する実装のはず
// swap は自作だから合っているか不明
void swap(long long x, long long y){
  long long tmp = 0;
  tmp = x;
  x = y;
  y = tmp;
}
// mod. m での a の逆元 a^{-1} を計算する
long long modinv(long long a, long long m) {
    long long b = m, u = 1, v = 0;
    while (b) {
        long long t = a / b;
        a -= t * b; swap(a, b);
        u -= t * v; swap(u, v);
    }
    u %= m;
    if (u < 0) u += m;
    return u;
    printf("ddd\n");
}

// http://drken1215.hatenablog.com/entry/2018/06/08/210000
// 典型的な二項係数の求め方 (1 ≦ k ≦ n ≦ 10^7 程度)
// nCk mod p の計算
// a <= k <= n <= 10^7
#define P_MAX 1000005 //配列の数が足りてなかった、、
long long fac[P_MAX], finv[P_MAX], inv[P_MAX];
// テーブルを作る前処理
void COMinit() {
  fac[0] = 1;
  fac[1] = 1;
  finv[0] = 1;
  finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < P_MAX; i++){
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(int n, int k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}


int compare_int(const void *a, const void *b){
  return *(long int*)b - *(long int*)a; // 大きい順
}
int compare_int_small(const void *a, const void *b){
  return *(long int*)a - *(long int*)b; // 小さい順
}
// 公約数
long long int gcd(long long int a, long long int b){
  if(b == 0){
    return a;
  }else{
    return gcd(b, a % b);
  }
}
// 最小公倍数 = a * b / 最大公約数
// 公倍数
long long int lcm(long long int a, long long int b){
  return a * b / gcd(a, b);
}

#define TRUE 1
#define FALSE 0
#define MAX_NUM 100002
typedef struct pp{
  char c;
  int h;
  int w;
}P;

int top = 0;
P *Stack = NULL;
P *tmp = NULL;


void initialize(){
  Stack = (P *)calloc(MAX_NUM, sizeof(P));
  top = 0;
}

void finalize(){
  if(Stack != NULL){
    free(Stack);
    Stack = NULL;
  }
}

int isEmpty(){
  if(top <= 0){
    return TRUE;
  }else{
    return FALSE;
  }
}

int isFull(){
  if(top > MAX_NUM - 1){
    return TRUE;
  }else{
    return FALSE;
  }
}

int push(P x){
  fprintf(stderr, "pushするのは%c (h, w) = (%d, %d)\n", x.c, x.h, x.w);
  //printf("push x = %d\n",x);
  if(isFull() != FALSE){
    //    fprintf(stderr, "pushしたのは%d top = %d\n", Stack[top], top);
    printf("Error! Overfllow\n");
    return 0;
  }else{
    top++;
    if(Stack != NULL){
      Stack[top] = x;
    }
    return 0;
  }
}

P pop(){
  if(isEmpty() != FALSE){
    printf("Error! Underfllow\n");
    return *tmp;
  }else{
    top--;
    // return では取りだす値を返す
    //    fprintf(stderr, "popしたのは%d top = %d\n", Stack[top + 1], top);
    return Stack[top+1];
  }
}

// min
int main(void){
  int x = 0, y = 0;
  long long top = 1;
  long long under = 1;
  long long ans = 0;
  long long a = 0, b = 0, c = 0;;

  scanf("%d%d", &x, &y);

  while(x > 0 || y > 0){
    if(x >= y){
      x -= 2;
      y -= 1;
      a++;
    }else{
      x -= 1;
      y -= 2;
      b++;
    }
  }

  if(x < 0 || y < 0){
    printf("0\n");
    return 0;
  }

  // テーブルを作る前処理
  COMinit();
  
  c = a + b;
  if(a > b){
    fprintf(stderr, "11111\n");
    // COM(n, k)
    ans = COM(c, b);
  }else{
    fprintf(stderr, "2222\n");
    // COM(n, k)
    ans = COM(c, a);
  }
  
  /*
  if(a > b){
    fprintf(stderr, "11111\n");
    for(int i = 0; i < b; i++){
      top = (top % MOD) * (a + b - i);
      under = under * (b - i);
    }
  }else{
    fprintf(stderr, "222222\n");
    for(int i = 0; i < a; i++){
      top = (top % MOD) * (a + b - i);
      under = under * (a - i);
      printf("i = %d top = %lld\n", i , top);
    }
  }

  top %= MOD;
  //  under = modinv(under, MOD);
  ans = (top / under) % MOD;
  */
  printf("%lld\n", ans);
  
  return 0;

}
/*
  int n = 0;
  int x[10];
  int y[10];
  double d[30500];
  double ans = 0;

  scanf("%d", &n);

  for(int i = 0; i < n; i++){
    scanf("%d%d", &x[i], y[i]);
  }

  d[0] = sqrt((x[i] - x[j])^2 + (y[i] - y[j])^2);
  printf("%f", ans);

 */