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

#include <bits/stdc++.h>
// ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
// clock_t start=clock();clock_t
// end=clock();cout<<(double)(end-start)/CLOCKS_PER_SEC<<endl;
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int ui;
typedef pair<int, int> pii;
typedef pair<pii, int> ppii;
typedef pair<int, pii> pipi;
typedef pair<ll, ll> pll;
typedef pair<pll, ll> ppll;
typedef pair<ll, pll> plpl;
typedef pair<pii, pii> pippi;
typedef tuple<ll, ll, ll> tl;
typedef pair<double, double> pdd;
typedef vector<vector<ll>> mat;
ll mod = 1000000007;
ll mod2 = 998244353;
ll mod3 = 1000003;
ll mod4 = 998244853;
ll mod5 = 1000000009;
ll inf = 1LL << 62;
int iinf = 1 << 30;
double pi = 3.14159265358979323846;
double pi2 = pi / 2.0;
double eps = 1e-8;
#define rep(i, m, n) for (ll i = m; i < n; i++)
#define rrep(i, n, m) for (ll i = n; i >= m; i--)
#define srep(itr, st) for (auto itr = st.begin(); itr != st.end(); itr++)
#define mrep(itr, mp) for (auto &itr : mp)
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
// #define endl "\\n"
int dh[4] = {1, -1, 0, 0};
int dw[4] = {0, 0, 1, -1};
int ddh[8] = {-1, -1, -1, 0, 0, 1, 1, 1};
int ddw[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    // http://xorshift.di.unimi.it/splitmix64.c
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
#define umh unordered_map<int, int, custom_hash>
ll gcd(ll a, ll b) {
  if (a < 0)
    a = -a;
  if (b < 0)
    b = -b;
  if (a < b)
    swap(a, b);
  if (b == 0)
    return a;
  if (a % b == 0)
    return b;
  return gcd(b, a % b);
}
ll lcm(ll a, ll b) {
  ll c = gcd(a, b);
  return a * b / c;
}
ll Pow(ll n, ll k) {
  if (k < 0)
    return 0;
  ll ret = 1;
  ll now = n;
  while (k > 0) {
    if (k & 1)
      ret *= now;
    now *= now;
    k /= 2;
  }
  return ret;
}
ll beki(ll n, ll k, ll md) {
  ll ret = 1;
  ll now = n;
  now %= md;
  while (k > 0) {
    if (k % 2 == 1) {
      ret *= now;
      ret %= md;
    }
    now *= now;
    now %= md;
    k /= 2;
  }
  return ret;
}
ll gyaku(ll n, ll md) { return beki(n, md - 2, md); }
ll popcount(ll n) {
  ll ret = 0;
  ll u = n;
  while (u > 0) {
    ret += u % 2;
    u /= 2;
  }
  return ret;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ll n;
  cin >> n;
  ll a[n];
  ll x = 0;
  rep(i, 0, n) {
    cin >> a[i];
    if (i >= 2)
      x ^= a[i];
  }
  ll s = a[0] + a[1];
  if (s - x < 0 || (s - x) % 2 == 1) {
    cout << -1 << endl;
    return 0;
  }
  ll d = (s - x) / 2;
  ll sum = 0;
  bool used[43];
  fill(used, used + 43, false);
  rep(i, 0, 43) {
    ll f = (x >> i) & 1, g = (d >> i) & 1;
    if (f == 1 && g == 1) {
      cout << -1 << endl;
      return 0;
    }
    if (g == 1) {
      sum += (1ll << i);
      used[i] = true;
    }
    if (f == 0 && g == 0)
      used[i] = true;
  }
  if (sum > a[0]) {
    cout << -1 << endl;
    return 0;
  }
  rrep(i, 42, 0) {
    if (used[i])
      continue;
    if (sum + (1ll << i) <= a[0])
      sum += (1ll << i);
  }
  if (sum == 0)
    cout << -1 << endl;
  else
    cout << a[0] - sum << endl;
}


Error: 0

Fixed submission:

#include <bits/stdc++.h>
// ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
// clock_t start=clock();clock_t
// end=clock();cout<<(double)(end-start)/CLOCKS_PER_SEC<<endl;
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int ui;
typedef pair<int, int> pii;
typedef pair<pii, int> ppii;
typedef pair<int, pii> pipi;
typedef pair<ll, ll> pll;
typedef pair<pll, ll> ppll;
typedef pair<ll, pll> plpl;
typedef pair<pii, pii> pippi;
typedef tuple<ll, ll, ll> tl;
typedef pair<double, double> pdd;
typedef vector<vector<ll>> mat;
ll mod = 1000000007;
ll mod2 = 998244353;
ll mod3 = 1000003;
ll mod4 = 998244853;
ll mod5 = 1000000009;
ll inf = 1LL << 61;
int iinf = 1 << 30;
double pi = 3.141592653589793238462643383279L;
double eps = 1e-8;
#define rep(i, m, n) for (ll i = m; i < n; i++)
#define rrep(i, n, m) for (ll i = n; i >= m; i--)
#define srep(itr, st) for (auto itr = st.begin(); itr != st.end(); itr++)
#define mrep(itr, mp) for (auto &itr : mp)
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
// #define endl "\\n"
int dh[4] = {1, -1, 0, 0};
int dw[4] = {0, 0, 1, -1};
int ddh[8] = {-1, -1, -1, 0, 0, 1, 1, 1};
int ddw[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    // http://xorshift.di.unimi.it/splitmix64.c
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
#define umh unordered_map<int, int, custom_hash>
ll gcd(ll a, ll b) {
  if (a < b)
    swap(a, b);
  if (b == 0)
    return a;
  if (a % b == 0)
    return b;
  return gcd(b, a % b);
}
ll lcm(ll a, ll b) {
  ll c = gcd(a, b);
  return a * b / c;
}
ll Pow(ll n, ll k) {
  if (k < 0)
    return 0;
  ll ret = 1;
  ll now = n;
  while (k > 0) {
    if (k & 1)
      ret *= now;
    now *= now;
    k /= 2;
  }
  return ret;
}
ll beki(ll n, ll k, ll md) {
  ll ret = 1;
  ll now = n;
  now %= md;
  while (k > 0) {
    if (k % 2 == 1) {
      ret *= now;
      ret %= md;
    }
    now *= now;
    now %= md;
    k /= 2;
  }
  return ret;
}
ll gyaku(ll n, ll md) { return beki(n, md - 2, md); }
ll popcount(ll n) {
  ll ret = 0;
  ll u = n;
  while (u > 0) {
    ret += u % 2;
    u /= 2;
  }
  return ret;
}
vector<pll> f(vector<pll> vv, vector<pll> a) {

  vector<pll> ret;
  rep(ii, 0, a.size()) {
    ll len = a[ii].second - a[ii].first + 1;
    rep(i, 0, vv.size()) {
      if (vv[i].second < a[ii].first)
        continue;
      ll sa = vv[i].first - a[ii].second;
      sa /= len;

      Max(sa, 0ll);

      rep(j, sa, sa + 10) {
        ll nl = a[ii].first + j * len, nr = a[ii].second + j * len;
        // cout<<nl<<" "<<nr<<" "<<vv[i].first<<" "<<vv[i].second<<endl;
        if (vv[i].first > nr)
          continue;
        if (vv[i].second < nl)
          break;
        ret.push_back({max(nl, vv[i].first), min(nr, vv[i].second)});
      }
    }
  }
  sort(ret.begin(), ret.end());
  ret.erase(unique(ret.begin(), ret.end()), ret.end());
  if (ret.size() == 0)
    return ret;
  vector<pll> rett;
  rett.push_back(ret[0]);
  rep(i, 1, ret.size()) {
    ll ri = rett.back().first;
    if (ret[i].first == ri) {
      rett.pop_back();
      rett.push_back(ret[i]);
    }
  }
  return rett;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ll n;
  cin >> n;
  ll a[n];
  rep(i, 0, n) cin >> a[i];
  ll sum = 0;
  rep(i, 2, n) sum ^= a[i];
  vector<pll> han;
  han.push_back({0, inf});
  rrep(i, 41, 0) {

    ll u = (a[0] >> i) & 1ll, v = (a[1] >> i) & 1ll, w = (sum >> i) & 1ll;
    ll k = 1ll << i;
    ll r0 = a[0] % k, r1 = a[1] % k;

    // if(i==2)cout<<u<<" "<<v<<" "<<w<<" "<<r0<<" "<<r1<<endl;
    if ((u ^ v) == w) {
      ll s1 = r0, s2 = k - r1 - 1;
      if (s1 == s2) {
        han = f(han, {{0, k - 1}});
        continue;
      }
      if (s1 < s2) {
        s2++;
      } else {
        swap(s1, s2);
        s2++;
      }
      // cout<<s1<<" "<<s2<<endl;
      han = f(han, {{0, s1}, {s2, k - 1}});
      //  cout<<han.size()<<endl;

    } else {
      ll s1 = r0, s2 = k - r1 - 1;
      if (s1 == s2) {
        cout << -1 << endl;
        return 0;
      }
      if (s1 < s2) {
        s1++;
      } else {
        swap(s1, s2);
        s1++;
      }
      han = f(han, {{s1, s2}});
    }
    if (han.size() == 0) {
      cout << -1 << endl;
      return 0;
    }
  }
  ll ans = inf;
  rep(i, 0, han.size()) {
    if (han[i].first >= a[0]) {
      continue;
    }
    Min(ans, han[i].first);
  }
  if (ans == inf)
    ans = -1;
  cout << ans << endl;
}