Problem p02782 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using VI = vector<int>;
using VL = vector<ll>;
template <class T> using PQ = priority_queue<T, vector<T>, greater<T>>;
#define FOR(i, a, n) for (int(i) = (a); (i) < (n); ++(i))
#define eFOR(i, a, n) for (int(i) = (a); (i) <= (n); ++(i))
#define rFOR(i, a, n) for (int(i) = (n)-1; (i) >= (a); --(i))
#define erFOR(i, a, n) for (int(i) = (n); (i) >= (a); --(i))
#define each(i, a) for (auto &i : a)
#define SORT(i) sort((i).begin(), (i).end())
#define rSORT(i, a) sort((i).begin(), (i).end(), (a))
#define all(i) (i).begin(), (i).end()
#define out(y, x) ((y) < 0 || h <= (y) || (x) < 0 || w <= (x))
#define line cout << "------------------------\n"
#define ENDL(i, n) ((i) == (n)-1 ? "\n" : " ")
#define stop system("pause") // comment out this on AOJ.
constexpr ll INF = 1000000000;
constexpr ll LLINF = 1LL << 60;
constexpr ll mod = 1000000007;
constexpr ll MOD = 998244353;
constexpr ld eps = 1e-10; // 1e-9?
constexpr ld pi = 3.1415926535897932;
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
inline void init() {
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}
template <class T> inline istream &operator>>(istream &is, vector<T> &v) {
  for (auto &elemnt : v)
    is >> elemnt;
  return is;
}
template <class T, class U>
inline istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> inline vector<T> vec(size_t a) { return vector<T>(a); }
template <class T> inline vector<T> defvec(T def, size_t a) {
  return vector<T>(a, def);
}
template <class T, class... Ts> inline auto vec(size_t a, Ts... ts) {
  return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...));
}
template <class T, class... Ts> inline auto defvec(T def, size_t a, Ts... ts) {
  return vector<decltype(defvec<T>(def, ts...))>(a, defvec<T>(def, ts...));
}

const int MAX = 510000;
ll fac[MAX], finv[MAX], inv[MAX];
bool Pre = true;
void cominit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  FOR(i, 2, MAX) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
ll com(int n, int k) {
  if (Pre) {
    cominit();
    Pre = false;
  }
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}
ll semisolve(int y, int x) { return com(y + x, x); }
ll solve(int y, int x) {
  ll ret = 0;
  eFOR(i, 0, y)(ret += semisolve(i + 1, x)) %= mod;
  return ret;
}

int main() {
  init();

  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  ll ans = solve(r2, c2);
  (ans -= solve(r1 - 1, c2) - mod) %= mod;
  (ans -= solve(r2, c1 - 1) - mod) %= mod;
  (ans += solve(r1 - 1, c1 - 1)) %= mod;
  cout << ans << "\n";
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using VI = vector<int>;
using VL = vector<ll>;
template <class T> using PQ = priority_queue<T, vector<T>, greater<T>>;
#define FOR(i, a, n) for (int(i) = (a); (i) < (n); ++(i))
#define eFOR(i, a, n) for (int(i) = (a); (i) <= (n); ++(i))
#define rFOR(i, a, n) for (int(i) = (n)-1; (i) >= (a); --(i))
#define erFOR(i, a, n) for (int(i) = (n); (i) >= (a); --(i))
#define each(i, a) for (auto &i : a)
#define SORT(i) sort((i).begin(), (i).end())
#define rSORT(i, a) sort((i).begin(), (i).end(), (a))
#define all(i) (i).begin(), (i).end()
#define out(y, x) ((y) < 0 || h <= (y) || (x) < 0 || w <= (x))
#define line cout << "------------------------\n"
#define ENDL(i, n) ((i) == (n)-1 ? "\n" : " ")
#define stop system("pause") // comment out this on AOJ.
constexpr ll INF = 1000000000;
constexpr ll LLINF = 1LL << 60;
constexpr ll mod = 1000000007;
constexpr ll MOD = 998244353;
constexpr ld eps = 1e-10; // 1e-9?
constexpr ld pi = 3.1415926535897932;
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
inline void init() {
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}
template <class T> inline istream &operator>>(istream &is, vector<T> &v) {
  for (auto &elemnt : v)
    is >> elemnt;
  return is;
}
template <class T, class U>
inline istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> inline vector<T> vec(size_t a) { return vector<T>(a); }
template <class T> inline vector<T> defvec(T def, size_t a) {
  return vector<T>(a, def);
}
template <class T, class... Ts> inline auto vec(size_t a, Ts... ts) {
  return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...));
}
template <class T, class... Ts> inline auto defvec(T def, size_t a, Ts... ts) {
  return vector<decltype(defvec<T>(def, ts...))>(a, defvec<T>(def, ts...));
}

const int MAX = 2000010;
ll fac[MAX], finv[MAX], inv[MAX];
bool Pre = true;
void cominit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  FOR(i, 2, MAX) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
ll com(int n, int k) {
  if (Pre) {
    cominit();
    Pre = false;
  }
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}
ll semisolve(int y, int x) { return com(y + x, x); }
ll solve(int y, int x) {
  ll ret = 0;
  eFOR(i, 0, y)(ret += semisolve(i + 1, x)) %= mod;
  return ret;
}

int main() {
  init();

  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  ll ans = solve(r2, c2);
  (ans -= solve(r1 - 1, c2) - mod) %= mod;
  (ans -= solve(r2, c1 - 1) - mod) %= mod;
  (ans += solve(r1 - 1, c1 - 1)) %= mod;
  cout << ans << "\n";
}