Problem p02697 (C++) — buggy submission, verdict: 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
#define SP << " " <<

typedef long long ll;
typedef pair<int, int> P;
typedef tuple<int, int, int> T;
typedef vector<int> vec;
typedef vector<P> pvec;
typedef vector<vector<int>> vvec;
typedef vector<vector<P>> pvvec;
typedef priority_queue<int> PQI;
typedef priority_queue<P> PQP;
typedef priority_queue<int, vector<int>, greater<int>> 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};
constexpr int MOD = (1000000007);
// const int MOD = (998244353);
// const int INF = (1 << 30); // 1073741824
const ll INF = (1LL << 60); // 1152921504606846976
const double PI = (3.141592653589794);
const double EPS = (0.0000000001); // 10^(-10)

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;
}
template <class T> inline T ceil(T a, T b) { return T((a + b - 1) / b); }
template <class T> inline T round(T a, T b) { return T(a / b); }
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> inline void out(T &a) {
  bool flag = true;
  for (auto &x : a) {
    if (flag) {
      flag = false;
    } else {
      cout << ' ';
    }
    cout << x;
  }
  cout << endl;
}

//----------------------------------------------------------------
int nmax = 200000; // 2*(10^5)
vvec G(nmax);

void solve4ts() {

  int n, m;
  cin >> n >> m;

  if (m == 1) {
    cout << "1 2" << endl;
    return;
  }

  int i1 = 1;
  int i2 = m + 2;

  for (int i = m; i >= 1; i--) {
    cout << i1 << " " << i1 + i << endl;
    i1++;
    swap(i1, i2);
  }
}
//-----------------------------------------------------------------

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(15);
  int repeat = 1;
  // cin >> repeat;
  while (repeat--)
    solve4ts();
}

Error: 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
#define SP << " " <<

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};
constexpr int MOD = (1000000007);
// const int MOD = (998244353);
// const int INF = (1 << 30); // 1073741824
const int INF = (1LL << 60); // 1152921504606846976
const double PI = (3.141592653589794);
const double EPS = (0.0000000001);

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;
}
template <class T> inline T ceil(T a, T b) { return T((a + b - 1) / b); }
template <class T> inline T round(T a, T b) { return T(a / b); }
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> inline void out(T &a) {
  bool flag = true;
  for (auto &x : a) {
    if (flag) {
      flag = false;
    } else {
      cout << ' ';
    }
    cout << x;
  }
  cout << endl;
}

//----------------------------------------------------------------
int nmax = 200000; // 2*(10^5)
vvec g(nmax);

void solve4ts() {

  int n, m;
  cin >> n >> m;
  if (n % 2 == 1) {
    vec a(2 * m);
    rep(i, 2 * m) { a[i] = i + 1; }
    vec b = a;
    reverse(all(b));
    rep(i, m) { cout << a[i] << " " << b[i] << endl; }
  } else {
    exit(1);
    vec a(2 * m);
    rep(i, 2 * m) { a[i] = i + 1; }
    vec b = a;
    reverse(all(b));
    if (m >= 2)
      swap(b[0], b[1]);
    rep(i, m) { cout << a[i] << " " << b[i] << endl; }
  }
}
//-----------------------------------------------------------------

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(15);
  int repeat = 1;
  // cin >> repeat;
  while (repeat--)
    solve4ts();
}