Problem p02782 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define rep(i, N) for (int i = 0; i < int(N); ++i)
#define rep1(i, N) for (int i = 1; i < int(N); ++i)
#define all(a) (a).begin(), (a).end()
#define print(v)                                                               \
  {                                                                            \
    cerr << #v << ": [ ";                                                      \
    for (auto _ : v)                                                           \
      cerr << _ << ", ";                                                       \
    cerr << "]" << endl;                                                       \
  }
#define printpair(v)                                                           \
  {                                                                            \
    cerr << #v << ": [ ";                                                      \
    for (auto _ : v)                                                           \
      cerr << "{" << _.first << "," << _.second << "}"                         \
           << ", ";                                                            \
    cerr << "]" << endl;                                                       \
  }
#define dump(x) cerr << #x << ": " << x << endl;
#define bit(k) (1LL << (k))
#define Yes "Yes"
#define No "No"
#define YES "YES"
#define NO "NO"
typedef long long ll;

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;
}

const int INF = (ll)1e9;
const ll INFLL = (ll)1e18 + 1;
const ll MOD = (ll)1e9 + 7;
const double PI = acos(-1.0);
/*
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
const string dir = "DRUL";
*/
struct mint {
  long long x;
  mint(long long _x = 0) : x((_x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint modpow(long long t) const {
    if (!t)
      return 1;
    mint a = modpow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime MOD
  mint inv() const { return modpow(MOD - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};
struct combination {
  vector<mint> fact, ifact;
  // constructor(initiation)
  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < MOD);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint Comb(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
  mint H(int n, int m) { return Comb(n + m, m); }
};

combination comb(1e6 + 1);
// (0, 0) ~ (a, b)までの和
// (2, 2)だったら、(3 + 3) C (3) - 1
mint f(int a, int b) { return comb.Comb(a + 1 + b + 1, a + 1) - 1; }
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(20);

  int a, b, c, d;
  cin >> a >> b >> c >> d;

  mint ans = f(c, d) - f(a - 1, d) - f(c, b - 1) + f(a - 1, b - 1);
  cout << ans.x << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define rep(i, N) for (int i = 0; i < int(N); ++i)
#define rep1(i, N) for (int i = 1; i < int(N); ++i)
#define all(a) (a).begin(), (a).end()
#define print(v)                                                               \
  {                                                                            \
    cerr << #v << ": [ ";                                                      \
    for (auto _ : v)                                                           \
      cerr << _ << ", ";                                                       \
    cerr << "]" << endl;                                                       \
  }
#define printpair(v)                                                           \
  {                                                                            \
    cerr << #v << ": [ ";                                                      \
    for (auto _ : v)                                                           \
      cerr << "{" << _.first << "," << _.second << "}"                         \
           << ", ";                                                            \
    cerr << "]" << endl;                                                       \
  }
#define dump(x) cerr << #x << ": " << x << endl;
#define bit(k) (1LL << (k))
#define Yes "Yes"
#define No "No"
#define YES "YES"
#define NO "NO"
typedef long long ll;

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;
}

const int INF = (ll)1e9;
const ll INFLL = (ll)1e18 + 1;
const ll MOD = (ll)1e9 + 7;
const double PI = acos(-1.0);
/*
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
const string dir = "DRUL";
*/
struct mint {
  long long x;
  mint(long long _x = 0) : x((_x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint modpow(long long t) const {
    if (!t)
      return 1;
    mint a = modpow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime MOD
  mint inv() const { return modpow(MOD - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};
struct combination {
  vector<mint> fact, ifact;
  // constructor(initiation)
  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < MOD);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint Comb(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
  mint H(int n, int m) { return Comb(n + m, m); }
};

combination comb(1e7 + 1);
// (0, 0) ~ (a, b)までの和
// (2, 2)だったら、(3 + 3) C (3) - 1
mint f(int a, int b) { return comb.Comb(a + 1 + b + 1, a + 1) - 1; }
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(20);

  int a, b, c, d;
  cin >> a >> b >> c >> d;

  mint ans = f(c, d) - f(a - 1, d) - f(c, b - 1) + f(a - 1, b - 1);
  cout << ans.x << endl;
}