Problem p02782 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;
using ll = long long;

#define MOD 1000000007
template <int mod> struct ModInt {
  int x;
  ModInt() : x(0) {}
  ModInt(long long y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}
  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }
  ModInt &operator^=(long long p) {
    ModInt res = 1;
    for (; p; p >>= 1) {
      if (p & 1)
        res *= *this;
      *this *= *this;
    }
    return *this = res;
  }
  ModInt operator-() const { return ModInt(-x); }
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }
  ModInt operator^(long long p) const { return ModInt(*this) ^= p; }
  bool operator==(const ModInt &p) const { return x == p.x; }
  bool operator!=(const ModInt &p) const { return x != p.x; }
  explicit operator int() const { return x; }
  ModInt operator=(const int p) {
    x = p;
    return ModInt(*this);
  }
  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      a -= t * b;
      std::swap(a, b);
      u -= t * v;
      std::swap(u, v);
    }
    return ModInt(u);
  }
  friend std::ostream &operator<<(std::ostream &os, const ModInt<mod> &p) {
    return os << p.x;
  }
  friend std::istream &operator>>(std::istream &is, ModInt<mod> &a) {
    long long x;
    is >> x;
    a = ModInt<mod>(x);
    return (is);
  }
};
typedef ModInt<MOD> mint;

struct MComb {
  std::vector<mint> fact;
  std::vector<mint> inversed;
  MComb(int n) { // O(n+log(mod))
    fact = std::vector<mint>(n + 1, 1);
    for (int i = 1; i <= n; i++)
      fact[i] = fact[i - 1] * mint(i);
    inversed = std::vector<mint>(n + 1);
    inversed[n] = fact[n] ^ (MOD - 2);
    for (int i = n - 1; i >= 0; i--)
      inversed[i] = inversed[i + 1] * mint(i + 1);
  }
  mint ncr(int n, int r) {
    if (n < r)
      return 0;
    if (n < 0 || r < 0)
      return 0;
    return (fact[n] * inversed[r] * inversed[n - r]);
  }
  mint npr(int n, int r) {
    if (n < r)
      return 0;
    if (n < 0 || r < 0)
      return 0;
    return (fact[n] * inversed[n - r]);
  }
  mint nhr(int n, int r) {
    assert(n + r - 1 < (int)fact.size());
    return ncr(n + r - 1, r);
  }
  mint fac(int n) {
    if (n < 0)
      return 0;
    return fact[n];
  }
};
MComb comb(500000);

mint F(int x, int y) {
  mint res = 0;
  for (int i = 0; i <= y; i++) {
    res += comb.ncr(x + i + 1, x);
  }
  return res;
}

int main() {
  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  c1--, r1--;

  cout << F(r2, c2) - F(r2, c1) - F(r1, c2) + F(r1, c1) << endl;
}

Fixed submission:
#include <bits/stdc++.h>

using namespace std;
using ll = long long;

#define MOD 1000000007
template <int mod> struct ModInt {
  int x;
  ModInt() : x(0) {}
  ModInt(long long y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}
  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }
  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    return *this;
  }
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }
  ModInt &operator^=(long long p) {
    ModInt res = 1;
    for (; p; p >>= 1) {
      if (p & 1)
        res *= *this;
      *this *= *this;
    }
    return *this = res;
  }
  ModInt operator-() const { return ModInt(-x); }
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }
  ModInt operator^(long long p) const { return ModInt(*this) ^= p; }
  bool operator==(const ModInt &p) const { return x == p.x; }
  bool operator!=(const ModInt &p) const { return x != p.x; }
  explicit operator int() const { return x; }
  ModInt operator=(const int p) {
    x = p;
    return ModInt(*this);
  }
  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      a -= t * b;
      std::swap(a, b);
      u -= t * v;
      std::swap(u, v);
    }
    return ModInt(u);
  }
  friend std::ostream &operator<<(std::ostream &os, const ModInt<mod> &p) {
    return os << p.x;
  }
  friend std::istream &operator>>(std::istream &is, ModInt<mod> &a) {
    long long x;
    is >> x;
    a = ModInt<mod>(x);
    return (is);
  }
};
typedef ModInt<MOD> mint;

struct MComb {
  std::vector<mint> fact;
  std::vector<mint> inversed;
  MComb(int n) { // O(n+log(mod))
    fact = std::vector<mint>(n + 1, 1);
    for (int i = 1; i <= n; i++)
      fact[i] = fact[i - 1] * mint(i);
    inversed = std::vector<mint>(n + 1);
    inversed[n] = fact[n] ^ (MOD - 2);
    for (int i = n - 1; i >= 0; i--)
      inversed[i] = inversed[i + 1] * mint(i + 1);
  }
  mint ncr(int n, int r) {
    if (n < r)
      return 0;
    if (n < 0 || r < 0)
      return 0;
    return (fact[n] * inversed[r] * inversed[n - r]);
  }
  mint npr(int n, int r) {
    if (n < r)
      return 0;
    if (n < 0 || r < 0)
      return 0;
    return (fact[n] * inversed[n - r]);
  }
  mint nhr(int n, int r) {
    assert(n + r - 1 < (int)fact.size());
    return ncr(n + r - 1, r);
  }
  mint fac(int n) {
    if (n < 0)
      return 0;
    return fact[n];
  }
};
MComb comb(5000000);

mint F(int x, int y) {
  mint res = 0;
  for (int i = 0; i <= y; i++) {
    res += comb.ncr(x + i + 1, x);
  }
  return res;
}

int main() {
  int r1, c1, r2, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  c1--, r1--;

  cout << F(r2, c2) - F(r2, c1) - F(r1, c2) + F(r1, c1) << endl;
}