Problem p02697 (C++) — buggy submission, verdict: Runtime Error

#define _USE_MATH_DEFINES
#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (n); ++i)
#define reps(i, n) for (int i = 1; i <= (n); ++i)
#define repd(i, n) for (int i = (n)-1; i >= 0; --i)
#define repds(i, n) for (int i = (n); i > 0; --i)
#define loop(i, x, n) for (int i = (x); i < (n); ++i)
#define loops(i, x, n) for (int i = (x); i <= (n); ++i)
#define loopd(i, x, n) for (int i = (x); i > (n); --i)
#define loopds(i, x, n) for (int i = (x); i >= (n); --i)
#define itrep(i, a) for (auto i = begin(a); i != end(a); ++i)
#define itrepd(i, a) for (auto i = rbegin(a); i != rend(a); ++i)
#define each(i, x) for (auto &i : x)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
template <class T>
using priority_queuel = priority_queue<T, vector<T>, greater<T>>;
constexpr ll inf = static_cast<ll>(1e18);
constexpr int iinf = static_cast<int>(1e9);
constexpr double dinf = 1e10;
constexpr ld ldinf = 1e18;
constexpr double eps = 1e-9;
ll mod = static_cast<ll>(1e9 + 7);
template <class T> constexpr int sz(const T &a) { return (int)a.size(); }
template <class T> constexpr T pow2(const T a) { return a * a; }
template <class T, class... Args> constexpr void mins(T &a, Args... args) {
  a = min<T>({a, args...});
}
template <class T, class... Args> constexpr void maxs(T &a, Args... args) {
  a = max<T>({a, args...});
}
template <class A, int N, class T> void fillArray(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}

int n, m;
void solve() {
  cin >> n >> m;
  auto s = n / 2;
  auto t = (n % 2 == 1 ? n : n - 1);
  rep(i, s) {
    auto l = i + 1;
    auto r = s - i;
    if (r <= l)
      break;
    if (r - l <= m)
      cout << l << " " << r << '\\n';
  }
  rep(i, n - s) {
    auto l = s + i + 1;
    auto r = t - i;
    if (r <= l)
      break;
    if (r - l <= m)
      cout << l << " " << r << '\\n';
  }
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  solve();
  cout.flush();
  return 0;
}


Error: WA

Fixed submission:

#define _USE_MATH_DEFINES
#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (n); ++i)
#define reps(i, n) for (int i = 1; i <= (n); ++i)
#define repd(i, n) for (int i = (n)-1; i >= 0; --i)
#define repds(i, n) for (int i = (n); i > 0; --i)
#define loop(i, x, n) for (int i = (x); i < (n); ++i)
#define loops(i, x, n) for (int i = (x); i <= (n); ++i)
#define loopd(i, x, n) for (int i = (x); i > (n); --i)
#define loopds(i, x, n) for (int i = (x); i >= (n); --i)
#define itrep(i, a) for (auto i = begin(a); i != end(a); ++i)
#define itrepd(i, a) for (auto i = rbegin(a); i != rend(a); ++i)
#define each(i, x) for (auto &i : x)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
template <class T>
using priority_queuel = priority_queue<T, vector<T>, greater<T>>;
constexpr ll inf = static_cast<ll>(1e18);
constexpr int iinf = static_cast<int>(1e9);
constexpr double dinf = 1e10;
constexpr ld ldinf = 1e18;
constexpr double eps = 1e-9;
ll mod = static_cast<ll>(1e9 + 7);
template <class T> constexpr int sz(const T &a) { return (int)a.size(); }
template <class T> constexpr T pow2(const T a) { return a * a; }
template <class T, class... Args> constexpr void mins(T &a, Args... args) {
  a = min<T>({a, args...});
}
template <class T, class... Args> constexpr void maxs(T &a, Args... args) {
  a = max<T>({a, args...});
}
template <class A, int N, class T> void fillArray(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}

int n, m;
bool used[100005];
void solve() {
  cin >> n >> m;
  int res = 0;
  rep(i, n) {
    if (used[i])
      continue;
    if (res >= m)
      break;
    ++res;
    if (i + res < n)
      used[i + res] = true;
    cout << i + 1 << " " << i + 1 + res << '\\n';
  }
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  solve();
  cout.flush();
  return 0;
}