Problem p02626 (C++) — buggy submission, verdict: Time Limit Exceeded

// #pragma GCC optimize("Ofast,no-stack-protector")
// #pragma GCC
// target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC optimize("Ofast")
/*#pragma GCC optimize("section-anchors")
#pragma GCC optimize("profile-values,profile-reorder-functions,tracer")
#pragma GCC optimize("vpt")
#pragma GCC optimize("rename-registers")
#pragma GCC optimize("move-loop-invariants")
#pragma GCC optimize("unswitch-loops")
#pragma GCC optimize("function-sections")
#pragma GCC optimize("data-sections")
#pragma GCC optimize("branch-target-load-optimize")
#pragma GCC optimize("branch-target-load-optimize2")
#pragma GCC optimize("btr-bb-exclusive")*/
// #pragma GCC optimize("inline")
// #pragma comment(linker, "/STACK:167077260")
#define _CRT_SECURE_NO_WARNINGS
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <string>
#include <time.h> //////////////
#include <unordered_map>
#include <unordered_set>
#include <vector>
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
// #define endl '\\n'
#define mp make_pair
#define pbc push_back
#define pob pop_back()
#define empb emplace_back
#define queuel queue<long long>
#define sqr(a) ((a) * (a))
#define lsqr(a) (ld(a) * a)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pin(p) cin >> p.first >> p.second;
#define uniq(a)                                                                \\
  sort(all(a));                                                                \\
  (a).resize(unique(all(a)) - a.begin());
#define rev(v) reverse(v.begin(), v.end());
#define sout(s, c)                                                             \\
  for (auto i : s)                                                             \\
    cout << i << c;
#define pout(p) cout << p.first << " " << p.second;
#define er(v, l, r) erase(v.begin() + l, v.begin() + r);
#define vin(v)                                                                 \\
  for (ll i = 0; i < v.size(); ++i)                                            \\
    cin >> v[i];
#define vout(v, c)                                                             \\
  for (int i = 0; i < v.size(); ++i)                                           \\
    cout << v[i] << c;
#define pushi(v, a)                                                            \\
  for (int i = 0; i < a.size(); ++i)                                           \\
    v.push_back(a[i]);
#define fastio()                                                               \\
  ios_base::sync_with_stdio(0);                                                \\
  cout.tie(0);                                                                 \\
  cin.tie(0);                                                                  \\
  srand(time(NULL))
#define dab(v)                                                                 \\
  for (auto i : v)                                                             \\
    cout << i << ' ';
#define sp system("pause")
#define left left228
#define type type1337
#define right right228
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
using namespace std;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
const ld EPS = 1e-5;
const ld EPS2 = 1e-12;
const ld PI = acos(-1);
int mod = (int)998244353;
const int MOD7 = 1000000007;
const int MOD9 = 1000000009;
const int a228 = 18;
const int inf = 1e9;
mt19937 rnd(time(0));
const ll kekmod = 1791791791;
const ll bestmod = 1148822869;
ll stupid(vector<ll> a) {
  ll ost = 0;
  for (int i = 2; i < a.size(); ++i)
    ost ^= a[i];
  for (int taken = 0; taken < a[0]; ++taken) {
    if ((a[0] - taken) ^ (a[0] + taken) == ost)
      return taken;
  }
  return -1;
}
signed main() {
  fastio();
  int n;
  cin >> n;
  vector<ll> a(n);
  vin(a);
  ll ost = 0;
  for (int i = 2; i < n; ++i)
    ost ^= a[i];
  ll ns = a[0] + a[1];
  ll nd = ns - ost;
  if (nd < 0 || nd % 2) {
    cout << -1;
    sp;
    return 0;
  }
  nd /= 2;
  for (int bt = 41; bt >= 0; --bt) {
    if (nd & (1ll << bt))
      continue;
    if ((nd | (1ll << bt)) <= a[0] && (ost & (1ll << bt))) {
      nd |= (1ll << bt);
    }
  }
  if (a[0] - nd >= a[0] || nd > a[0]) {
    cout << -1;
    sp;
    return 0;
  }
  if (nd ^ (ns - nd) ^ ost) {
    cout << -1;
    sp;
    return 0;
  }
  cout << a[0] - nd;
  sp;
}

Error: TLE

Fixed submission:

// #pragma GCC optimize("Ofast,no-stack-protector")
// #pragma GCC
// target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC optimize("Ofast")
/*#pragma GCC optimize("section-anchors")
#pragma GCC optimize("profile-values,profile-reorder-functions,tracer")
#pragma GCC optimize("vpt")
#pragma GCC optimize("rename-registers")
#pragma GCC optimize("move-loop-invariants")
#pragma GCC optimize("unswitch-loops")
#pragma GCC optimize("function-sections")
#pragma GCC optimize("data-sections")
#pragma GCC optimize("branch-target-load-optimize")
#pragma GCC optimize("branch-target-load-optimize2")
#pragma GCC optimize("btr-bb-exclusive")*/
// #pragma GCC optimize("inline")
// #pragma comment(linker, "/STACK:167077260")
#define _CRT_SECURE_NO_WARNINGS
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <string>
#include <time.h> //////////////
#include <unordered_map>
#include <unordered_set>
#include <vector>
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
// #define endl '\\n'
#define mp make_pair
#define pbc push_back
#define pob pop_back()
#define empb emplace_back
#define queuel queue<long long>
#define sqr(a) ((a) * (a))
#define lsqr(a) (ld(a) * a)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pin(p) cin >> p.first >> p.second;
#define uniq(a)                                                                \\
  sort(all(a));                                                                \\
  (a).resize(unique(all(a)) - a.begin());
#define rev(v) reverse(v.begin(), v.end());
#define sout(s, c)                                                             \\
  for (auto i : s)                                                             \\
    cout << i << c;
#define pout(p) cout << p.first << " " << p.second;
#define er(v, l, r) erase(v.begin() + l, v.begin() + r);
#define vin(v)                                                                 \\
  for (ll i = 0; i < v.size(); ++i)                                            \\
    cin >> v[i];
#define vout(v, c)                                                             \\
  for (int i = 0; i < v.size(); ++i)                                           \\
    cout << v[i] << c;
#define pushi(v, a)                                                            \\
  for (int i = 0; i < a.size(); ++i)                                           \\
    v.push_back(a[i]);
#define fastio()                                                               \\
  ios_base::sync_with_stdio(0);                                                \\
  cout.tie(0);                                                                 \\
  cin.tie(0);                                                                  \\
  srand(time(NULL))
#define dab(v)                                                                 \\
  for (auto i : v)                                                             \\
    cout << i << ' ';
#define sp system("pause")
#define left left228
#define type type1337
#define right right228
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
using namespace std;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
const ld EPS = 1e-5;
const ld EPS2 = 1e-12;
const ld PI = acos(-1);
int mod = (int)998244353;
const int MOD7 = 1000000007;
const int MOD9 = 1000000009;
const int a228 = 18;
const int inf = 1e9;
mt19937 rnd(time(0));
const ll kekmod = 1791791791;
const ll bestmod = 1148822869;
signed main() {
  fastio();
  int n;
  cin >> n;
  vector<ll> a(n);
  vin(a);
  ll ost = 0;
  for (int i = 2; i < n; ++i)
    ost ^= a[i];
  ll ns = a[0] + a[1];
  ll nd = ns - ost;
  if (nd < 0 || nd % 2) {
    cout << -1;
    sp;
    return 0;
  }
  nd /= 2;
  for (int bt = 41; bt >= 0; --bt) {
    if (nd & (1ll << bt))
      continue;
    if ((nd | (1ll << bt)) <= a[0]) {
      nd |= (1ll << bt);
    }
  }
  if (a[0] - nd >= a[0] || nd > a[0]) {
    cout << -1;
    sp;
    return 0;
  }
  if (nd ^ (ns - nd) ^ ost)
    while (1)
      ;
  cout << a[0] - nd;
  sp;
}