Problem p02862 (C++)

Buggy submission (status: Runtime Error):
// includes
#include "bits/stdc++.h"
using namespace std;
// macros
#define ll long long
#define MOD 1000000007 // 998244353 //100000000  //
#define pii pair<ll, ll>
#define piii pair<ll, pii>
#define sz(x) ((ll)(x).size())
#define ft first
#define sd second
#define pb push_back
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repr(i, n) for (ll i = n - 1; i >= 0; i--)
#define itr(it, x) for (auto it = x.begin(); it != x.end(); it++)
#define mem(a, b) memset(a, (ll)b, sizeof(a))
#define all(v) v.begin(), v.end()
#define allr(v) v.rbegin(), v.rend()
#define edge(v, x, y)                                                          \
  v[x].pb(y);                                                                  \
  v[y].pb(x);
#define popc __builtin_popcount
#define ANS(s)                                                                 \
  {                                                                            \
    cout << s << "\n";                                                         \
    return;                                                                    \
  }
// functions
template <typename T> void unique(T &c) {
  c.erase(std::unique(c.begin(), c.end()), c.end());
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec)
    is >> v;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i];
    if (i + 1 != vec.size())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << "(" << itr->first << ", " << itr->second << ")";
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << "(" << itr->first << ", " << itr->second << ")";
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
// constants
const ll N = 1e5 + 5, M = 1e6 + 5, A = 1e7 + 5, inf = 1e9;
const long long linf = 1LL << 60;
const double er = 1e-10, pi = 3.141592653589793238463;
const ll lx[4] = {0, 1, -1, 0};
const ll ly[4] = {1, 0, 0, -1};
const ll dx[8] = {0, 0, 1, -1, 1, -1, 1, -1};
const ll dy[8] = {1, -1, 0, 0, 1, 1, -1, -1};
// io
struct fast_io {
  fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(20);
  }
} fast_io_;

ll f[M], fin[M];
ll power(ll a, ll b = MOD - 2) {
  if (b == 0)
    return 1;
  ll r = power(a, b / 2);
  r = (r * r) % MOD;
  if (b % 2)
    r = (r * a) % MOD;
  return r;
}
void pre() {
  f[0] = fin[0] = 1;
  for (ll i = 1; i < M; i++) {
    f[i] = (f[i - 1] * i) % MOD;
    fin[i] = (fin[i - 1] * power(i)) % MOD;
  }
}
ll C(ll n, ll r) { return (f[n] * ((fin[r] * fin[n - r]) % MOD)) % MOD; }
void solve() {
  pre();
  ll x, y;
  cin >> x >> y;
  if ((x + y) % 3 != 0)
    ANS(0);
  ll k = (x + y) / 3;
  ll cx = k * 2, cy = k;
  if (x > cx || y < cy)
    ANS(0);
  ll no = abs(cx - x) + abs(cy - y);
  no /= 2; // cout << cx << " " << cy << endl;

  cout << C(k, no);
}
int main(int argc, char const *argv[]) {
  ll t = 1; // cin >> t;
  while (t--) {
    solve();
  }
  return 0;
}


Fixed submission:
// includes
#include "bits/stdc++.h"
using namespace std;
// macros
#define ll long long
#define MOD 1000000007 // 998244353 //100000000  //
#define pii pair<ll, ll>
#define piii pair<ll, pii>
#define sz(x) ((ll)(x).size())
#define ft first
#define sd second
#define pb push_back
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repr(i, n) for (ll i = n - 1; i >= 0; i--)
#define itr(it, x) for (auto it = x.begin(); it != x.end(); it++)
#define mem(a, b) memset(a, (ll)b, sizeof(a))
#define all(v) v.begin(), v.end()
#define allr(v) v.rbegin(), v.rend()
#define edge(v, x, y)                                                          \
  v[x].pb(y);                                                                  \
  v[y].pb(x);
#define popc __builtin_popcount
#define ANS(s)                                                                 \
  {                                                                            \
    cout << s << "\n";                                                         \
    return;                                                                    \
  }
// functions
template <typename T> void unique(T &c) {
  c.erase(std::unique(c.begin(), c.end()), c.end());
}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec)
    is >> v;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i];
    if (i + 1 != vec.size())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << "(" << itr->first << ", " << itr->second << ")";
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << "(" << itr->first << ", " << itr->second << ")";
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
// constants
const ll N = 1e5 + 5, M = 1e6 + 5, A = 1e7 + 5, inf = 1e9;
const long long linf = 1LL << 60;
const double er = 1e-10, pi = 3.141592653589793238463;
const ll lx[4] = {0, 1, -1, 0};
const ll ly[4] = {1, 0, 0, -1};
const ll dx[8] = {0, 0, 1, -1, 1, -1, 1, -1};
const ll dy[8] = {1, -1, 0, 0, 1, 1, -1, -1};
// io
struct fast_io {
  fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(20);
  }
} fast_io_;

ll f[M], fin[M];
ll power(ll a, ll b = MOD - 2) {
  if (b == 0)
    return 1;
  ll r = power(a, b / 2);
  r = (r * r) % MOD;
  if (b % 2)
    r = (r * a) % MOD;
  return r;
}
void pre() {
  f[0] = fin[0] = 1;
  for (ll i = 1; i < M; i++) {
    f[i] = (f[i - 1] * i) % MOD;
    fin[i] = (fin[i - 1] * power(i)) % MOD;
  }
}
ll C(ll n, ll r) { return (f[n] * ((fin[r] * fin[n - r]) % MOD)) % MOD; }
void solve() {
  pre();
  ll x, y;
  cin >> x >> y;
  if ((x + y) % 3 != 0)
    ANS(0);
  ll k = (x + y) / 3;
  ll cx = k * 2, cy = k;
  if (x > cx || x < cy)
    ANS(0);
  ll no = abs(cx - x) + abs(cy - y);
  no /= 2; // cout << cx << " " << cy << endl;

  cout << C(k, no);
}
int main(int argc, char const *argv[]) {
  ll t = 1; // cin >> t;
  while (t--) {
    solve();
  }
  return 0;
}