Problem p03090 (C++)

Buggy submission (status: Runtime Error):
// #define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define REP(i, n) for (int i = 0; i < n; ++i)
#define range(i, a, b) ((a) <= (i) && (i) < (b))
#define debug(x) cout << #x << ' ' << '=' << ' ' << (x) << endl;
#define fs first
#define sc second
#define pb push_back
#define eb emplace_back
typedef long long ll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> T;
typedef vector<ll> vec;
typedef vector<P> pvec;
typedef vector<vector<ll>> vvec;
typedef vector<vector<P>> pvvec;
typedef priority_queue<ll> PQI;
typedef priority_queue<P> PQP;
typedef priority_queue<ll, vector<ll>, greater<ll>> PQIG;
typedef priority_queue<P, vector<P>, greater<P>> PQPG;
const vector<int> dx = {0, -1, 0, 1, 1, 1, -1, -1};
const vector<int> dy = {1, 0, -1, 0, 1, -1, 1, -1};
const int MOD = (1000000007);
// const int MOD = (998244353);
// const int INF = (1 << 30);
const int INF = (1LL << 60);
const double EPS = (1 >> 30);
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(15);
  //---------------------------------------------

  int n;
  cin >> n;
  if (n == 3) {
    cout << "2 1 3 2 3" << endl;
  } else if (n % 2 == 0) {
    int memo = n + 1;
    int cnt = 0;
    rep(i, n - 1) {
      rep(j, n) {
        if (i + 1 + j + 1 == memo || i >= j)
          continue;
        cnt++;
      }
    }
    cout << cnt << endl;
    rep(i, n - 1) {
      rep(j, n) {
        if (i + 1 + j + 1 == memo || i >= j)
          continue;
        cout << i + 1 << " " << j + 1 << endl;
      }
    }
  } else {
    exit(1);
  }
}

Fixed submission:
// #define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define REP(i, n) for (int i = 0; i < n; ++i)
#define range(i, a, b) ((a) <= (i) && (i) < (b))
#define debug(x) cout << #x << ' ' << '=' << ' ' << (x) << endl;
#define fs first
#define sc second
#define pb push_back
#define eb emplace_back
typedef long long ll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> T;
typedef vector<ll> vec;
typedef vector<P> pvec;
typedef vector<vector<ll>> vvec;
typedef vector<vector<P>> pvvec;
typedef priority_queue<ll> PQI;
typedef priority_queue<P> PQP;
typedef priority_queue<ll, vector<ll>, greater<ll>> PQIG;
typedef priority_queue<P, vector<P>, greater<P>> PQPG;
const vector<int> dx = {0, -1, 0, 1, 1, 1, -1, -1};
const vector<int> dy = {1, 0, -1, 0, 1, -1, 1, -1};
const int MOD = (1000000007);
// const int MOD = (998244353);
// const int INF = (1 << 30);
const int INF = (1LL << 60);
const double EPS = (1 >> 30);
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(15);
  //---------------------------------------------

  int n;
  cin >> n;
  if (n == 3) {
    cout << "2 1 3 2 3" << endl;
  } else if (n % 2 == 0) {
    int memo = n + 1;
    int cnt = 0;
    rep(i, n - 1) {
      rep(j, n) {
        if (i + 1 + j + 1 == memo || i >= j)
          continue;
        cnt++;
      }
    }
    cout << cnt << endl;
    rep(i, n - 1) {
      rep(j, n) {
        if (i + 1 + j + 1 == memo || i >= j)
          continue;
        cout << i + 1 << " " << j + 1 << endl;
      }
    }
  } else {
    // exit(1);
    pvec a(n / 2 + 1);
    for (int i = 0; i < n / 2; i++) {
      a[i] = P(i + 1, n - (i + 1));
      // cout<<i+1<<n-i-1<<endl;
    }
    a[n / 2] = P(n, -1);
    int x = a.size();
    int cnt = 0;
    for (int i = 0; i < x - 1; i++) {
      for (int j = i + 1; j < x; j++) {
        // cout<<a[i].fs<<" "<<a[j].fs<<endl;
        // cout<<a[i].sc<<" "<<a[j].fs<<endl;
        cnt += 2;
        if (j == x - 1)
          continue;
        // cout<<a[i].fs<<" "<<a[j].sc<<endl;
        // cout<<a[i].sc<<" "<<a[j].sc<<endl;
        cnt += 2;
      }
    }
    cout << cnt << endl;
    for (int i = 0; i < x - 1; i++) {
      for (int j = i + 1; j < x; j++) {
        cout << a[i].fs << " " << a[j].fs << endl;
        cout << a[i].sc << " " << a[j].fs << endl;
        // cnt+=2;
        if (j == x - 1)
          continue;
        cout << a[i].fs << " " << a[j].sc << endl;
        cout << a[i].sc << " " << a[j].sc << endl;
        // cnt+=2;
      }
    }
  }
}