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

#include <bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(), x.end()
#define rep(i, n) for (int i = 0; i < (n); i++)
#define debug(x)                                                               \\
  cout << #x << ":";                                                           \\
  cout << x << endl;
#define debugs(v)                                                              \\
  cout << #v << ":";                                                           \\
  for (auto x : v) {                                                           \\
    cout << x << ' ';                                                          \\
  }                                                                            \\
  cout << endl;
#define INF 1000000000
#define mod 1000000007
using ll = long long;
const ll LINF = 1001002003004005006ll;
int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(0);

  ll n, m;
  cin >> n >> m;
  if (n % 2) {
    ll l = 1, r = n - 1;
    rep(i, m) {
      cout << l << " " << r << endl;
      l++, r--;
    }
    return 0;
  } else {
    ll l = 1, r = n;
    if ((n / 2) % 2) {
      rep(i, m) {
        cout << l << " " << r << endl;
        l++, r--;
        if (r - l == n / 2) {
          r--;
        }
      }
    } else {
      ll l = 1, r = n;
      set<pair<ll, ll>> st;
      bool f = true;
      rep(i, m) {
        cout << l << " " << r << endl;
        l++, r--;
        ll x = r - l;
        ll y = n - x;
        if (f and st.count(make_pair(max(x, y), min(x, y)))) {
          r--;
          f = false;
        }
        st.insert(make_pair(max(x, y), min(x, y)));
      }
    }
    return 0;
  }

  return 0;
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(), x.end()
#define rep(i, n) for (int i = 0; i < (n); i++)
#define debug(x)                                                               \\
  cout << #x << ":";                                                           \\
  cout << x << endl;
#define debugs(v)                                                              \\
  cout << #v << ":";                                                           \\
  for (auto x : v) {                                                           \\
    cout << x << ' ';                                                          \\
  }                                                                            \\
  cout << endl;
#define INF 1000000000
#define mod 1000000007
using ll = long long;
const ll LINF = 1001002003004005006ll;
int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(0);

  ll n, m;
  cin >> n >> m;
  if (n % 2) {
    ll l = 1, r = n - 1;
    rep(i, m) {
      cout << l << " " << r << endl;
      l++, r--;
    }
    return 0;
  } else {
    ll l = 1, r = n;
    rep(i, m) {
      cout << l << " " << r << endl;
      l++, r--;
      if (r - l == n / 2) {
        r--;
        assert(false);
      }
    }
    return 0;
  }

  return 0;
}