Problem p02782 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
template <class A, class B>
ostream &operator<<(ostream &out, const pair<A, B> &p) {
  return out << "(" << p.first << ", " << p.second << ")";
}
template <typename T, size_t N>
ostream &operator<<(ostream &out, const array<T, N> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T, class Cmp>
ostream &operator<<(ostream &out, const set<T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename U, typename T, class Cmp>
ostream &operator<<(ostream &out, const map<U, T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &p : a) {
    out << (first ? "" : ", ");
    out << p.first << ":" << p.second;
    first = 0;
  }
  out << "}";
  return out;
}
template <class T> inline void YES(T condition) {
  if (condition)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
template <class T> inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T> T gcd(T a, T b) {
  if (a && b) {
    return gcd(min(a, b), max(a, b) % min(a, b));
  } else {
    return a;
  }
}
template <class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

long long qpow(long long a, long long b, int MOD) {
  if (b == 0)
    return 1;
  long long res = 1;
  while (b) {
    if (b & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    b >>= 1;
  }
  return res;
}
const int MOD = 1e9 + 7;
const int maxn = 1e6;
int fac[maxn + 5];
int invfac[maxn + 5];
void init() {
  fac[0] = 1;
  for (int i = 1; i <= maxn; i++)
    fac[i] = 1LL * fac[i - 1] * i % MOD;
  invfac[maxn] = (int)(qpow(fac[maxn], MOD - 2, MOD));
  for (int i = maxn - 1; i >= 0; i--)
    invfac[i] = 1LL * invfac[i + 1] * (i + 1) % MOD;
}
int C(int n, int k) {
  assert(0 <= k && n >= k);
  int tmp = (int)(1LL * invfac[k] * invfac[n - k] % MOD);
  return (int)(1LL * fac[n] * tmp % MOD);
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);

  init();
  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  LL ans = C(r1 + c1, r1);
  for (int i = r1 + 1; i <= r2; i++)
    ans = (ans + C(c2 + i, c2)) % MOD;
  for (int j = c1 + 1; j <= c2; j++)
    ans = (ans + C(r2 + j, j)) % MOD;
  ans = ((ans % MOD) + MOD) % MOD;
  cout << ans << '\n';

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
template <class A, class B>
ostream &operator<<(ostream &out, const pair<A, B> &p) {
  return out << "(" << p.first << ", " << p.second << ")";
}
template <typename T, size_t N>
ostream &operator<<(ostream &out, const array<T, N> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T, class Cmp>
ostream &operator<<(ostream &out, const set<T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename U, typename T, class Cmp>
ostream &operator<<(ostream &out, const map<U, T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &p : a) {
    out << (first ? "" : ", ");
    out << p.first << ":" << p.second;
    first = 0;
  }
  out << "}";
  return out;
}
template <class T> inline void YES(T condition) {
  if (condition)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
template <class T> inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T> T gcd(T a, T b) {
  if (a && b) {
    return gcd(min(a, b), max(a, b) % min(a, b));
  } else {
    return a;
  }
}
template <class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

long long qpow(long long a, long long b, int MOD) {
  if (b == 0)
    return 1;
  long long res = 1;
  while (b) {
    if (b & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    b >>= 1;
  }
  return res;
}
const int MOD = 1e9 + 7;
const int maxn = 2e6;
int fac[maxn + 5];
int invfac[maxn + 5];
void init() {
  fac[0] = 1;
  for (int i = 1; i <= maxn; i++)
    fac[i] = 1LL * fac[i - 1] * i % MOD;
  invfac[maxn] = (int)(qpow(fac[maxn], MOD - 2, MOD));
  for (int i = maxn - 1; i >= 0; i--)
    invfac[i] = 1LL * invfac[i + 1] * (i + 1) % MOD;
}
int C(int n, int k) {
  assert(0 <= k && n >= k);
  int tmp = (int)(1LL * invfac[k] * invfac[n - k] % MOD);
  return (int)(1LL * fac[n] * tmp % MOD);
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);

  init();
  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  LL ans = C(r1 + c1, r1);
  for (int i = r1 + 1; i <= r2; i++)
    ans = (ans + C(c2 + i, c2)) % MOD;
  for (int j = c1 + 1; j <= c2; j++)
    ans = (ans + C(r2 + j, j)) % MOD;
  ans = ((ans % MOD) + MOD) % MOD;
  cout << ans << '\n';

  return 0;
}