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

/*                         _
                        _ooOoo_
                       o8888888o
                       88" . "88
                       (| -_- |)
                       O\\  =  /O
                    ____/`---'\\____
                  .'  \\\\|     |//  `.
                 /  \\\\|||  :  |||//  \\
                /  _||||| -:- |||||_  \\
                |   | \\\\\\  -  /'| |   |
                | \\_|  `\\`---'//  |_/ |
                \\  .-\\__ `-. -'__/-.  /
              ___`. .'  /--.--\\  `. .'___
           ."" '<  `.___\\_<|>_/___.' _> \\"".
          | | :  `- \\`. ;`. _/; .'/ /  .' ; |
          \\  \\ `-.   \\_\\_`. _.'_/_/  -' _.' /
===========`-.`___`-.__\\ \\___  /__.-'_.'_.-'================

                  Please give me AC.
*/

#include <algorithm>
#include <bitset>
#include <chrono>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using int64 = long long;
using uint64 = unsigned long long;
using vi = vector<int>;
using vl = vector<int64>;
using pii = pair<int, int>;
using pll = pair<int64, int64>;

#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define all(v) (v).begin(), (v).end()
#define print(x) cout << (x) << '\\n'
#define print2(x, y) cout << (x) << ' ' << (y) << '\\n'
#define print3(x, y, z) cout << (x) << ' ' << (y) << ' ' << (z) << '\\n'
#define printn(v)                                                              \\
  rep(i, (v).size() - 1) cout << (v)[i] << ' ';                                \\
  cout << (v)[n - 1] << '\\n';
#define debug(x) cerr << #x << ": " << (x) << '\\n'
#define debug2(x, y)                                                           \\
  cerr << #x << ": " << (x) << ", " << #y << ": " << (y) << '\\n'
#define debug3(x, y, z)                                                        \\
  cerr << #x << ": " << (x) << ", " << #y << ": " << (y) << ", " << #z << ": " \\
       << (z) << '\\n'
#define dbg(v)                                                                 \\
  for (size_t _ = 0; _ < v.size(); ++_) {                                      \\
    cerr << #v << "[" << _ << "] : " << v[_] << '\\n';                          \\
  }
// constant
const int INF = (1 << 30) - 1;
const int64 INF64 = (1LL << 62) - 1;

template <typename T> T gcd(T a, T b) {
  if (a < b)
    return gcd(b, a);
  T r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

template <typename T> T lcm(const T a, const T b) { return a / gcd(a, b) * b; }

template <typename T> bool chmin(T &a, const T &b) {
  if (a > b)
    return a = b, true;
  else
    return false;
}

template <typename T> bool chmax(T &a, const T &b) {
  if (a < b)
    return a = b, true;
  else
    return false;
}

// End of template.

int main() {
  cout << fixed << setprecision(15);
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  // cin.read(buf, sizeof buf); // 注意: ./a.out < in か pbp | ./a.out
  // で入力すること

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

  if (n & 1) {
    rep(i, m) { print2(i + 1, n - i - 1); }
  } else {
    int n2 = n / 2;
    int a = 1, b = n2;
    while (b - a >= 1 && m-- > 0) {
      print2(a, b);
      ++a, --b;
    }
    a = n2 + 1;
    b = n - 1;
    while (b - a >= 1 && m-- > 0) {
      print2(a, b);
      ++a, --b;
    }
  }

  return 0;
}


Error: 1

Fixed submission:

/*                         _
                        _ooOoo_
                       o8888888o
                       88" . "88
                       (| -_- |)
                       O\\  =  /O
                    ____/`---'\\____
                  .'  \\\\|     |//  `.
                 /  \\\\|||  :  |||//  \\
                /  _||||| -:- |||||_  \\
                |   | \\\\\\  -  /'| |   |
                | \\_|  `\\`---'//  |_/ |
                \\  .-\\__ `-. -'__/-.  /
              ___`. .'  /--.--\\  `. .'___
           ."" '<  `.___\\_<|>_/___.' _> \\"".
          | | :  `- \\`. ;`. _/; .'/ /  .' ; |
          \\  \\ `-.   \\_\\_`. _.'_/_/  -' _.' /
===========`-.`___`-.__\\ \\___  /__.-'_.'_.-'================

                  Please give me AC.
*/

#include <algorithm>
#include <bitset>
#include <chrono>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using int64 = long long;
using uint64 = unsigned long long;
using vi = vector<int>;
using vl = vector<int64>;
using pii = pair<int, int>;
using pll = pair<int64, int64>;

#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define all(v) (v).begin(), (v).end()
#define print(x) cout << (x) << '\\n'
#define print2(x, y) cout << (x) << ' ' << (y) << '\\n'
#define print3(x, y, z) cout << (x) << ' ' << (y) << ' ' << (z) << '\\n'
#define printn(v)                                                              \\
  rep(i, (v).size() - 1) cout << (v)[i] << ' ';                                \\
  cout << (v)[n - 1] << '\\n';
#define debug(x) cerr << #x << ": " << (x) << '\\n'
#define debug2(x, y)                                                           \\
  cerr << #x << ": " << (x) << ", " << #y << ": " << (y) << '\\n'
#define debug3(x, y, z)                                                        \\
  cerr << #x << ": " << (x) << ", " << #y << ": " << (y) << ", " << #z << ": " \\
       << (z) << '\\n'
#define dbg(v)                                                                 \\
  for (size_t _ = 0; _ < v.size(); ++_) {                                      \\
    cerr << #v << "[" << _ << "] : " << v[_] << '\\n';                          \\
  }
// constant
const int INF = (1 << 30) - 1;
const int64 INF64 = (1LL << 62) - 1;

template <typename T> T gcd(T a, T b) {
  if (a < b)
    return gcd(b, a);
  T r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

template <typename T> T lcm(const T a, const T b) { return a / gcd(a, b) * b; }

template <typename T> bool chmin(T &a, const T &b) {
  if (a > b)
    return a = b, true;
  else
    return false;
}

template <typename T> bool chmax(T &a, const T &b) {
  if (a < b)
    return a = b, true;
  else
    return false;
}

// End of template.

int main() {
  cout << fixed << setprecision(15);
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  // cin.read(buf, sizeof buf); // 注意: ./a.out < in か pbp | ./a.out
  // で入力すること

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

  if (n & 1) {
    rep(i, m) { print2(i + 1, n - i - 1); }
  } else {
    return 1;
    for (int i = 1; i < m / 2; ++i) {
      print2(i, n / 2 - i);
    }
    for (int i = m / 2; i <= m; ++i) {
      print2(i, n - i);
    }
  }

  return 0;
}