Problem p02862 (C++)

Buggy submission (status: Runtime Error):
// BROWNIE TK

#include <bits/stdc++.h>
typedef long long int lli;
typedef unsigned long long int ulli;
typedef long double ldb;

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

#define pb push_back
#define popb pop_back()
#define pf push_front
#define popf pop_front()
#define si size()
#define be begin()
#define en end()
#define all(v) v.be, v.en
#define le length()
#define mp make_pair
#define mt make_tuple
#define acc(v) accumulate(all(v), 0)
#define F first
#define S second

#define forz(i, n) for (int i = 0; i < n; i++)
#define fore(i, m, n) for (int i = m; i <= n; i++)
#define rforz(i, n) for (int i = n - 1; i >= 0; i--)
#define rforzm(i, m, n) for (int i = n - 1; i >= m; i--)
#define deci(n) fixed << setprecision(n)
#define high(n) __builtin_popcount(n)
#define highll(n) __builtin_popcountll(n)
#define parity(n) __builtin_parity(n)
#define ctz(n) __builtin_ctz(n)
#define lb lower_bound
#define ub upper_bound
#define er equal_range
#define maxe *max_element
#define mine *min_element
#define mod 1000000007
#define mod2 998244353
#define gcd __gcd
#define kira ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)

#define endl "\n"
#define p0(a) cout << a << " "
#define p1(a) cout << a << endl
#define p2(a, b) cout << a << " " << b << endl
#define p3(a, b, c) cout << a << " " << b << " " << c << endl
#define p4(a, b, c, d) cout << a << " " << b << " " << c << " " << d << endl

#define oset                                                                   \
  tree<int, null_type, less<int>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>
#define osetlli                                                                \
  tree<lli, null_type, less<lli>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>
// member functions :
// 1. order_of_key(k) : number of elements strictly lesser than k
// 2. find_by_order(k) : k-th element in the set
#define ofk order_of_key
#define fbo find_by_order
using namespace std;

/*STD fucntions*/
lli power(lli x, lli y, lli p) {
  lli res = 1;
  x = x % p;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % p;
    // y must be even now
    y = y >> 1; // y=y/2
    x = (x * x) % p;
  }
  return res;
}
lli modi(lli a, lli m) {
  // fermat little thm where m is prime
  return power(a, m - 2, m);
}
/*CODE BEGINS*/
const int N = 1e6 + 5;
lli fac[N];
int main() {
  kira;
  lli x, y;
  fac[0] = 1;
  for (int i = 1; i < N; i++) {
    fac[i] = (fac[i - 1] * i) % mod;
  }
  cin >> x >> y;
  lli s = x + y;
  lli a, b;
  if (s % 3)
    cout << 0;
  else {
    lli c = s / 3;
    b = x - c;
    if (b < 0)
      cout << 0;
    else {
      lli ans = fac[c];
      ans *= modi(fac[b], mod);
      ans %= mod;
      ans *= modi(fac[c - b], mod);
      ans %= mod;
      cout << ans;
    }
  }
  return 0;
}

Fixed submission:
// BROWNIE TK

#include <bits/stdc++.h>
typedef long long int lli;
typedef unsigned long long int ulli;
typedef long double ldb;

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

#define pb push_back
#define popb pop_back()
#define pf push_front
#define popf pop_front()
#define si size()
#define be begin()
#define en end()
#define all(v) v.be, v.en
#define le length()
#define mp make_pair
#define mt make_tuple
#define acc(v) accumulate(all(v), 0)
#define F first
#define S second

#define forz(i, n) for (int i = 0; i < n; i++)
#define fore(i, m, n) for (int i = m; i <= n; i++)
#define rforz(i, n) for (int i = n - 1; i >= 0; i--)
#define rforzm(i, m, n) for (int i = n - 1; i >= m; i--)
#define deci(n) fixed << setprecision(n)
#define high(n) __builtin_popcount(n)
#define highll(n) __builtin_popcountll(n)
#define parity(n) __builtin_parity(n)
#define ctz(n) __builtin_ctz(n)
#define lb lower_bound
#define ub upper_bound
#define er equal_range
#define maxe *max_element
#define mine *min_element
#define mod 1000000007
#define mod2 998244353
#define gcd __gcd
#define kira ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)

#define endl "\n"
#define p0(a) cout << a << " "
#define p1(a) cout << a << endl
#define p2(a, b) cout << a << " " << b << endl
#define p3(a, b, c) cout << a << " " << b << " " << c << endl
#define p4(a, b, c, d) cout << a << " " << b << " " << c << " " << d << endl

#define oset                                                                   \
  tree<int, null_type, less<int>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>
#define osetlli                                                                \
  tree<lli, null_type, less<lli>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>
// member functions :
// 1. order_of_key(k) : number of elements strictly lesser than k
// 2. find_by_order(k) : k-th element in the set
#define ofk order_of_key
#define fbo find_by_order
using namespace std;

/*STD fucntions*/
lli power(lli x, lli y, lli p) {
  lli res = 1;
  x = x % p;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % p;
    // y must be even now
    y = y >> 1; // y=y/2
    x = (x * x) % p;
  }
  return res;
}
lli modi(lli a, lli m) {
  // fermat little thm where m is prime
  return power(a, m - 2, m);
}
/*CODE BEGINS*/
const int N = 1e6 + 5;
lli fac[N];
int main() {
  kira;
  lli x, y;
  fac[0] = 1;
  for (int i = 1; i < N; i++) {
    fac[i] = (fac[i - 1] * i) % mod;
  }
  cin >> x >> y;
  lli s = x + y;
  lli a, b;
  if (s % 3)
    cout << 0;
  else {
    lli c = s / 3;
    b = x - c;
    if (b < 0 || b > c)
      cout << 0;
    else {
      lli ans = fac[c];
      ans *= modi(fac[b], mod);
      ans %= mod;
      ans *= modi(fac[c - b], mod);
      ans %= mod;
      cout << ans;
    }
  }
  return 0;
}