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

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rrep(i, n) for (int i = n - 1; i >= 0; i--)
#define rep2(i, s, n) for (int i = s; i < n; ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define tmax(a, b, c) max(a, max(b, c))
#define tmin(a, b, c) min(a, min(b, c))
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<P>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (T t : v) {
    os << t << " ";
  }
  os << "\\n";
  return os;
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YES(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const int inf = 1001001001;
const ll linf = 1001001001001001001;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int n, m;
  cin >> n >> m;
  int l = 1, r = 1 + m;
  while (l < r) {
    cout << l << " " << r << endl;
    l++;
    r--;
  }
  l = m + 2, r = l + m - 1;
  while (l < r) {
    cout << l << " " << r << endl;
    l++;
    r--;
  }
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rrep(i, n) for (int i = n - 1; i >= 0; i--)
#define rep2(i, s, n) for (int i = s; i < n; ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define tmax(a, b, c) max(a, max(b, c))
#define tmin(a, b, c) min(a, min(b, c))
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<P>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (T t : v) {
    os << t << " ";
  }
  os << "\\n";
  return os;
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YES(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const int inf = 1001001001;
const ll linf = 1001001001001001001;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int n, m;
  cin >> n >> m;
  vb used(n, false);
  int now = 0;
  rep2(i, 1, m + 1) {
    while (used[now])
      now++;
    used[now] = true;
    used[now + i] = true;
    cout << now + 1 << " " << now + i + 1 << endl;
  }
}