Problem p03268 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define countof(array) (sizeof(array) / sizeof(array[0]))
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, n, s) for (int i = s; i < (n); ++i)
#define rsrep(i, n, s) for (int i = (n)-1; i >= s; --i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define aall(a) (a), (a) + countof(a) // for array sorting
#define raall(a) (a), (a) + countof(a), greater<>()
#define show(x) cout << #x << " = " << x << endl;
#define vfind(v, a) find(all(v), a) != v.end()
#define yn(f)                                                                  \
  {                                                                            \
    if (f)                                                                     \
      puts("YES");                                                             \
    else                                                                       \
      puts("NO");                                                              \
  }
#define yns(f)                                                                 \
  {                                                                            \
    if (f)                                                                     \
      puts("Yes");                                                             \
    else                                                                       \
      puts("No");                                                              \
  }
#define show_ary(...)                                                          \
  {                                                                            \
    cout << #__VA_ARGS__ << " = ";                                             \
    for (const auto &x : (__VA_ARGS__)) {                                      \
      cout << x << " ";                                                        \
    }                                                                          \
    cout << endl;                                                              \
  }
#define show_pair(...)                                                         \
  cout << #__VA_ARGS__ << " = " << endl;                                       \
  for (const auto &x : (__VA_ARGS__)) {                                        \
    cout << "  " << x.fi << " : " << x.se << endl;                             \
  }
#define out_ary(...)                                                           \
  {                                                                            \
    for (const auto &x : (__VA_ARGS__)) {                                      \
      cout << x << " ";                                                        \
    }                                                                          \
    cout << endl;                                                              \
  }
#define argmax(v) distance((v).begin(), max_element(all(v)))
#define argmin(v) distance((v).begin(), min_element(all(v)))
#define vmax(v) *max_element(all(v))
#define vmin(v) *min_element(all(v))
typedef long long int ll;
typedef pair<int, int> P;
typedef vector<P> vpair;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<double> vdouble;
typedef vector<string> vstr;
typedef vector<bool> vbool;
typedef vector<vint> vvint;
typedef vector<vll> vvll;
typedef vector<vstr> vvstr;
typedef vector<vbool> vvbool;
const ll LINF = 2000000000000000000ll;
const int INF = 1000000100;
const ll MOD = 1e9 + 7;

int main() {
  int n, k;
  cin >> n >> k;
  vll m(n);
  rep(i, n) m[(i + 1) % k]++;
  ll ans = 0;
  rep(a, k) {
    int tmp = (k - a) % k;
    int b = tmp, c = tmp;
    if ((b + c) % k == 0)
      ans += m[a] * m[b] * m[c];
  }

  cout << ans << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define countof(array) (sizeof(array) / sizeof(array[0]))
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, n, s) for (int i = s; i < (n); ++i)
#define rsrep(i, n, s) for (int i = (n)-1; i >= s; --i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define aall(a) (a), (a) + countof(a) // for array sorting
#define raall(a) (a), (a) + countof(a), greater<>()
#define show(x) cout << #x << " = " << x << endl;
#define vfind(v, a) find(all(v), a) != v.end()
#define yn(f)                                                                  \
  {                                                                            \
    if (f)                                                                     \
      puts("YES");                                                             \
    else                                                                       \
      puts("NO");                                                              \
  }
#define yns(f)                                                                 \
  {                                                                            \
    if (f)                                                                     \
      puts("Yes");                                                             \
    else                                                                       \
      puts("No");                                                              \
  }
#define show_ary(...)                                                          \
  {                                                                            \
    cout << #__VA_ARGS__ << " = ";                                             \
    for (const auto &x : (__VA_ARGS__)) {                                      \
      cout << x << " ";                                                        \
    }                                                                          \
    cout << endl;                                                              \
  }
#define show_pair(...)                                                         \
  cout << #__VA_ARGS__ << " = " << endl;                                       \
  for (const auto &x : (__VA_ARGS__)) {                                        \
    cout << "  " << x.fi << " : " << x.se << endl;                             \
  }
#define out_ary(...)                                                           \
  {                                                                            \
    for (const auto &x : (__VA_ARGS__)) {                                      \
      cout << x << " ";                                                        \
    }                                                                          \
    cout << endl;                                                              \
  }
#define argmax(v) distance((v).begin(), max_element(all(v)))
#define argmin(v) distance((v).begin(), min_element(all(v)))
#define vmax(v) *max_element(all(v))
#define vmin(v) *min_element(all(v))
typedef long long int ll;
typedef pair<int, int> P;
typedef vector<P> vpair;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<double> vdouble;
typedef vector<string> vstr;
typedef vector<bool> vbool;
typedef vector<vint> vvint;
typedef vector<vll> vvll;
typedef vector<vstr> vvstr;
typedef vector<vbool> vvbool;
const ll LINF = 2000000000000000000ll;
const int INF = 1000000100;
const ll MOD = 1e9 + 7;

int main() {
  int n, k;
  cin >> n >> k;
  vll m(k, 0);
  rep(i, n) m[(i + 1) % k]++;
  ll ans = 0;
  rep(a, k) {
    int tmp = (k - a) % k;
    int b = tmp, c = tmp;
    if ((b + c) % k == 0)
      ans += m[a] * m[b] * m[c];
  }

  cout << ans << endl;
  return 0;
}