Problem p02782 (C++)

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

#define pb push_back
#define eb emplace_back
#define mp make_pair
#define ll long long
#define ull unsigned long long
#define F first
#define S second
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }

template <typename T, typename V> void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T> void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x)
    cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V> void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v))
    cerr << ", ";
  _print(v...);
}
#ifndef ONLINE_JUDGE
#define debug(x...)                                                            \
  cerr << "[" << #x << "] = [";                                                \
  _print(x)
#else
#define debug(x...)
#endif
const int MOD = 1e9 + 7;
const int N = 1e6;
ll iv[3 * N], f1[3 * N], f2[3 * N];

ll combi(int n, int k) { return f1[n] * f2[k] % MOD * f2[n - k] % MOD; }

int main(void) {

#ifndef LOCAL
  freopen("input.txt", "r", stdin);
#endif

  iv[1] = 1;
  for (int i = 2; i < 3 * N; i++)
    iv[i] = MOD - MOD / i * iv[MOD % i] % MOD;
  f1[0] = f2[0] = 1;
  for (int i = 1; i < 3 * N; i++) {
    f1[i] = f1[i - 1] * i % MOD;
    f2[i] = f2[i - 1] * iv[i] % MOD;
  }

  int r1, r2, c1, c2;
  scanf("%d %d %d %d", &r1, &c1, &r2, &c2);

  int ans = 0;
  cout << (combi(r2 + c2 + 2, c2 + 1) - combi(r1 + c2 + 1, c2 + 1) -
           combi(r2 + c1 + 1, c1) + combi(r1 + c1, c1) % MOD + MOD) %
              MOD;

  // cout << fixed << setprecision(9) << (double) (ans+k)*0.5 << '\n';

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

#define pb push_back
#define eb emplace_back
#define mp make_pair
#define ll long long
#define ull unsigned long long
#define F first
#define S second
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }

template <typename T, typename V> void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T> void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x)
    cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V> void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v))
    cerr << ", ";
  _print(v...);
}
#ifndef ONLINE_JUDGE
#define debug(x...)                                                            \
  cerr << "[" << #x << "] = [";                                                \
  _print(x)
#else
#define debug(x...)
#endif
const int MOD = 1e9 + 7;
const int N = 1e6;
ll iv[3 * N], f1[3 * N], f2[3 * N];

ll combi(int n, int k) { return f1[n] * f2[k] % MOD * f2[n - k] % MOD; }

int main(void) {

#ifndef ONLINE_JUDGE
// freopen("input.txt", "r", stdin);
#endif

  iv[1] = 1;
  for (int i = 2; i < 3 * N; i++)
    iv[i] = MOD - MOD / i * iv[MOD % i] % MOD;
  f1[0] = f2[0] = 1;
  for (int i = 1; i < 3 * N; i++) {
    f1[i] = f1[i - 1] * i % MOD;
    f2[i] = f2[i - 1] * iv[i] % MOD;
  }

  int r1, r2, c1, c2;
  scanf("%d %d %d %d", &r1, &c1, &r2, &c2);

  int ans = 0;
  cout << (combi(r2 + c2 + 2, c2 + 1) - combi(r1 + c2 + 1, c2 + 1) -
           combi(r2 + c1 + 1, c1) + combi(r1 + c1, c1) % MOD + MOD) %
              MOD;

  // cout << fixed << setprecision(9) << (double) (ans+k)*0.5 << '\n';

  return 0;
}