Problem p02742 (C++)

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

#define debug(x)                                                               \
  cout << "DEBUG"                                                              \
       << " " << #x << ":" << x << '\n'

// n回繰り返し
#define rep(i, n) for (int i = 0; i < ((int)(n)); i++)     // 0-indexed昇順
#define rep1(i, n) for (int i = 1; i <= ((int)(n)); i++)   // 1-indexed昇順
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; i--) // 0-indexed降順
#define rrep1(i, n) for (int i = ((int)(n)); i >= 1; i--)  // 1-indexed降順
#define all(x) (x).begin(), (x).end()

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<vi> vii; // 2次元配列
typedef vector<ll> vll;
typedef vector<double> vd;
typedef pair<int, int> PII;

template <typename T> using vec = vector<T>;
template <class T> using maxheap = std::priority_queue<T>;
template <class T>
using minheap = std::priority_queue<T, std::vector<T>, std::greater<T>>;

template <class T, class U> inline bool chmax(T &a, const U &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &a, const U &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

const ld Pi = std::acos(-1.0L);
const ll infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;
const int mod = 1000000007;
// const int mod = 998244353;

/*FUNCs=================================================*/

/*MAIN==================================================*/
signed main() {
  int _StartTime_ = clock();
  cin.tie(nullptr);
  ios::sync_with_stdio(false); // cin cout 高速化
  // cout << fixed << setprecision(15);

  ll h, w;
  cin >> h >> w;

  ll ans = 0;
  if (w == 1 || h == 1) {
    cout << 1 << '\n';
    return 1;
  }
  rep(i, h) {
    if (i % 2 == 0)
      ans += (w + 1) / 2;
    else
      ans += w / 2;
  }
  cout << ans << '\n';

  // printf("ExecutionTime: %d
  // /ms\n",1000*(int)((clock()-_StartTime_)/CLOCKS_PER_SEC));
}

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

#define debug(x)                                                               \
  cout << "DEBUG"                                                              \
       << " " << #x << ":" << x << '\n'

// n回繰り返し
#define rep(i, n) for (int i = 0; i < ((int)(n)); i++)     // 0-indexed昇順
#define rep1(i, n) for (int i = 1; i <= ((int)(n)); i++)   // 1-indexed昇順
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; i--) // 0-indexed降順
#define rrep1(i, n) for (int i = ((int)(n)); i >= 1; i--)  // 1-indexed降順
#define all(x) (x).begin(), (x).end()

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<vi> vii; // 2次元配列
typedef vector<ll> vll;
typedef vector<double> vd;
typedef pair<int, int> PII;

template <typename T> using vec = vector<T>;
template <class T> using maxheap = std::priority_queue<T>;
template <class T>
using minheap = std::priority_queue<T, std::vector<T>, std::greater<T>>;

template <class T, class U> inline bool chmax(T &a, const U &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &a, const U &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

const ld Pi = std::acos(-1.0L);
const ll infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;
const int mod = 1000000007;
// const int mod = 998244353;

/*FUNCs=================================================*/

/*MAIN==================================================*/
signed main() {
  int _StartTime_ = clock();
  cin.tie(nullptr);
  ios::sync_with_stdio(false); // cin cout 高速化
  // cout << fixed << setprecision(15);

  ll h, w;
  cin >> h >> w;

  ll ans = 0;
  if (w == 1 || h == 1) {
    cout << 1 << '\n';
    return 0;
  }
  rep(i, h) {
    if (i % 2 == 0)
      ans += (w + 1) / 2;
    else
      ans += w / 2;
  }
  cout << ans << '\n';

  // printf("ExecutionTime: %d
  // /ms\n",1000*(int)((clock()-_StartTime_)/CLOCKS_PER_SEC));
}