Problem p02626 (C++) — buggy submission, verdict: Runtime Error

// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
#define int ll
#define double ld
#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 repR(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep1R(i, n) for (int i = (int)(n); i >= 1; i--)
#define loop(i, a, B) for (int i = a; i B; i++)
#define loopR(i, a, B) for (int i = a; i B; i--)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fst first
#define snd second
template <class Int> auto constexpr inf = numeric_limits<Int>::max() / 2 - 1;
auto constexpr INF32 = inf<int32_t>;
auto constexpr INF64 = inf<int64_t>;
auto constexpr INF = inf<int>;
#ifdef LOCAL
#include "debug.hpp"
#else
#define dump(...) (void)(0)
#define say(x) (void)(0)
#define debug if (0)
#endif
template <class T> using pque_max = priority_queue<T>;
template <class T> using pque_min = priority_queue<T, vector<T>, greater<T>>;
template <class T, class = typename T::iterator,
          class = typename enable_if<!is_same<T, string>::value>::type>
ostream &operator<<(ostream &os, T const &v) {
  bool f = true;
  for (auto const &x : v)
    os << (f ? "" : " ") << x, f = false;
  return os;
}
template <class T, class = typename T::iterator,
          class = typename enable_if<!is_same<T, string>::value>::type>
istream &operator>>(istream &is, T &v) {
  for (auto &x : v)
    is >> x;
  return is;
}
template <class T, class S>
ostream &operator<<(ostream &os, pair<T, S> const &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <class T, class S> istream &operator>>(istream &is, pair<T, S> &p) {
  return is >> p.first >> p.second;
}
struct IOSetup {
  IOSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);
  }
} iosetup;
template <class F> struct FixPoint : private F {
  constexpr FixPoint(F &&f) : F(forward<F>(f)) {}
  template <class... T> constexpr auto operator()(T &&...x) const {
    return F::operator()(*this, forward<T>(x)...);
  }
};
struct MakeFixPoint {
  template <class F> constexpr auto operator|(F &&f) const {
    return FixPoint<F>(forward<F>(f));
  }
};
#define MFP MakeFixPoint() |
#define def(name, ...) auto name = MFP[&](auto &&name, __VA_ARGS__)
template <class T, size_t d> struct vec_impl {
  using type = vector<typename vec_impl<T, d - 1>::type>;
  template <class... U> static type make_v(size_t n, U &&...x) {
    return type(n, vec_impl<T, d - 1>::make_v(forward<U>(x)...));
  }
};
template <class T> struct vec_impl<T, 0> {
  using type = T;
  static type make_v(T const &x = {}) { return x; }
};
template <class T, size_t d = 1> using vec = typename vec_impl<T, d>::type;
template <class T, size_t d = 1, class... Args> auto make_v(Args &&...args) {
  return vec_impl<T, d>::make_v(forward<Args>(args)...);
}
template <class T> void quit(T const &x) {
  cout << x << endl;
  exit(0);
}
template <class T, class U> constexpr bool chmin(T &x, U const &y) {
  if (x > y) {
    x = y;
    return true;
  }
  return false;
}
template <class T, class U> constexpr bool chmax(T &x, U const &y) {
  if (x < y) {
    x = y;
    return true;
  }
  return false;
}
template <class It> constexpr auto sumof(It b, It e) {
  return accumulate(b, e, typename iterator_traits<It>::value_type{});
}
template <class T> int sz(T const &x) { return x.size(); }
template <class C, class T> int lbd(C const &v, T const &x) {
  return lower_bound(v.begin(), v.end(), x) - v.begin();
}
template <class C, class T> int ubd(C const &v, T const &x) {
  return upper_bound(v.begin(), v.end(), x) - v.begin();
}
template <class C, class F> int ppt(C const &v, F f) {
  return partition_point(v.begin(), v.end(), f) - v.begin();
}
// <<<

int32_t main() {
  int n;
  cin >> n;
  vector<int> a(n);
  cin >> a;

  int A = a[0] + a[1];
  int B = 0;
  loop(i, 2, < n) B ^= a[i];
  if (!(A >= B && (A - B) % 2 == 0))
    quit(-1);

  int C = (A - B) / 2;

  int X = 0;
  const int LG = 45;
  rep(i, LG) {
    int b = 0;
    if ((B >> i & 1) == 0 && (C >> i & 1) == 0)
      b = 0;
    if ((B >> i & 1) == 1 && (C >> i & 1) == 0)
      b = 0; //
    if ((B >> i & 1) == 0 && (C >> i & 1) == 1)
      b = 1;
    if ((B >> i & 1) == 1 && (C >> i & 1) == 1)
      quit(-1);
    X ^= (b << i);
  }
  if (X > a[0])
    quit(-1);

  repR(i, LG) {
    if ((B >> i & 1) == 1 && (C >> i & 1) == 0 && (X ^ (1LL << i)) <= a[0]) {
      X ^= (1LL << i);
    }
  }
  if (X == 0)
    quit(-1);

  cout << a[0] - X << endl;
}


Error: 0

Fixed submission:

// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
#define int ll
#define double ld
#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 repR(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep1R(i, n) for (int i = (int)(n); i >= 1; i--)
#define loop(i, a, B) for (int i = a; i B; i++)
#define loopR(i, a, B) for (int i = a; i B; i--)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fst first
#define snd second
template <class Int> auto constexpr inf = numeric_limits<Int>::max() / 2 - 1;
auto constexpr INF32 = inf<int32_t>;
auto constexpr INF64 = inf<int64_t>;
auto constexpr INF = inf<int>;
#ifdef LOCAL
#include "debug.hpp"
#else
#define dump(...) (void)(0)
#define say(x) (void)(0)
#define debug if (0)
#endif
template <class T> using pque_max = priority_queue<T>;
template <class T> using pque_min = priority_queue<T, vector<T>, greater<T>>;
template <class T, class = typename T::iterator,
          class = typename enable_if<!is_same<T, string>::value>::type>
ostream &operator<<(ostream &os, T const &v) {
  bool f = true;
  for (auto const &x : v)
    os << (f ? "" : " ") << x, f = false;
  return os;
}
template <class T, class = typename T::iterator,
          class = typename enable_if<!is_same<T, string>::value>::type>
istream &operator>>(istream &is, T &v) {
  for (auto &x : v)
    is >> x;
  return is;
}
template <class T, class S>
ostream &operator<<(ostream &os, pair<T, S> const &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <class T, class S> istream &operator>>(istream &is, pair<T, S> &p) {
  return is >> p.first >> p.second;
}
struct IOSetup {
  IOSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);
  }
} iosetup;
template <class F> struct FixPoint : private F {
  constexpr FixPoint(F &&f) : F(forward<F>(f)) {}
  template <class... T> constexpr auto operator()(T &&...x) const {
    return F::operator()(*this, forward<T>(x)...);
  }
};
struct MakeFixPoint {
  template <class F> constexpr auto operator|(F &&f) const {
    return FixPoint<F>(forward<F>(f));
  }
};
#define MFP MakeFixPoint() |
#define def(name, ...) auto name = MFP[&](auto &&name, __VA_ARGS__)
template <class T, size_t d> struct vec_impl {
  using type = vector<typename vec_impl<T, d - 1>::type>;
  template <class... U> static type make_v(size_t n, U &&...x) {
    return type(n, vec_impl<T, d - 1>::make_v(forward<U>(x)...));
  }
};
template <class T> struct vec_impl<T, 0> {
  using type = T;
  static type make_v(T const &x = {}) { return x; }
};
template <class T, size_t d = 1> using vec = typename vec_impl<T, d>::type;
template <class T, size_t d = 1, class... Args> auto make_v(Args &&...args) {
  return vec_impl<T, d>::make_v(forward<Args>(args)...);
}
template <class T> void quit(T const &x) {
  cout << x << endl;
  exit(0);
}
template <class T, class U> constexpr bool chmin(T &x, U const &y) {
  if (x > y) {
    x = y;
    return true;
  }
  return false;
}
template <class T, class U> constexpr bool chmax(T &x, U const &y) {
  if (x < y) {
    x = y;
    return true;
  }
  return false;
}
template <class It> constexpr auto sumof(It b, It e) {
  return accumulate(b, e, typename iterator_traits<It>::value_type{});
}
template <class T> int sz(T const &x) { return x.size(); }
template <class C, class T> int lbd(C const &v, T const &x) {
  return lower_bound(v.begin(), v.end(), x) - v.begin();
}
template <class C, class T> int ubd(C const &v, T const &x) {
  return upper_bound(v.begin(), v.end(), x) - v.begin();
}
template <class C, class F> int ppt(C const &v, F f) {
  return partition_point(v.begin(), v.end(), f) - v.begin();
}
// <<<

int32_t main() {
  int n;
  cin >> n;
  vector<int> a(n);
  cin >> a;

  int A = a[0] + a[1];
  int B = 0;
  loop(i, 2, < n) B ^= a[i];
  if (!(A >= B && (A - B) % 2 == 0))
    quit(-1);

  int C = (A - B) / 2;

  int X = 0;
  const int LG = 45;
  rep(i, LG) {
    int b = 0;
    if ((B >> i & 1) == 0 && (C >> i & 1) == 0)
      b = 0;
    if ((B >> i & 1) == 1 && (C >> i & 1) == 0)
      b = 0; //
    if ((B >> i & 1) == 0 && (C >> i & 1) == 1)
      b = 1;
    if ((B >> i & 1) == 1 && (C >> i & 1) == 1)
      assert(0);
    X ^= (b << i);
  }

  repR(i, LG) {
    if ((B >> i & 1) == 1 && (C >> i & 1) == 0 && (X ^ (1LL << i)) <= a[0]) {
      X ^= (1LL << i);
    }
  }
  if (X == 0)
    quit(-1);

  cout << a[0] - X << endl;
}