Problem p02997 (C++)

Buggy submission (status: Time Limit Exceeded):
// need
#include <algorithm>
#include <iostream>
// data structure
#include <bitset>
#include <complex>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
// #include <deque>
#include <array>
#include <unordered_map>
#include <unordered_set>
#include <valarray>
// stream
// #include <istream>
#include <sstream>
// #include <ostream>
#include <fstream>
// etc
#include <cassert>
#include <chrono>
#include <cmath>
#include <functional>
#include <iomanip>
#include <numeric>
#include <random>

// input
#define INIT                                                                   \
  std::ios::sync_with_stdio(false);                                            \
  std::cin.tie(0);
#define VAR(type, ...)                                                         \
  type __VA_ARGS__;                                                            \
  MACRO_VAR_Scan(__VA_ARGS__);
template <typename T> void MACRO_VAR_Scan(T &t) { std::cin >> t; }
template <typename First, typename... Rest>
void MACRO_VAR_Scan(First &first, Rest &...rest) {
  std::cin >> first;
  MACRO_VAR_Scan(rest...);
}
#define VEC_ROW(type, n, ...)                                                  \
  std::vector<type> __VA_ARGS__;                                               \
  MACRO_VEC_ROW_Init(n, __VA_ARGS__);                                          \
  for (int w_ = 0; w_ < n; ++w_) {                                             \
    MACRO_VEC_ROW_Scan(w_, __VA_ARGS__);                                       \
  }
template <typename T> void MACRO_VEC_ROW_Init(int n, T &t) { t.resize(n); }
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Init(int n, First &first, Rest &...rest) {
  first.resize(n);
  MACRO_VEC_ROW_Init(n, rest...);
}
template <typename T> void MACRO_VEC_ROW_Scan(int p, T &t) { std::cin >> t[p]; }
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Scan(int p, First &first, Rest &...rest) {
  std::cin >> first[p];
  MACRO_VEC_ROW_Scan(p, rest...);
}
#define VEC(type, c, n)                                                        \
  std::vector<type> c(n);                                                      \
  for (auto &i : c)                                                            \
    std::cin >> i;
#define MAT(type, c, m, n)                                                     \
  std::vector<std::vector<type>> c(m, std::vector<type>(n));                   \
  for (auto &R : c)                                                            \
    for (auto &w : R)                                                          \
      std::cin >> w;
// output
#define OUT(dist) std::cout << (dist);
#define FOUT(n, dist) std::cout << std::fixed << std::setprecision(n) << (dist);
#define SOUT(n, c, dist) std::cout << std::setw(n) << std::setfill(c) << (dist);
#define SP std::cout << " ";
#define TAB std::cout << "\t";
#define BR std::cout << "\n";
#define SPBR(w, n) std::cout << (w + 1 == n ? '\n' : ' ');
#define ENDL std::cout << std::endl;
#define FLUSH std::cout << std::flush;
#define SHOW(dist)                                                             \
  { std::cerr << #dist << "\t:" << (dist) << "\n"; }
#define SHOWVECTOR(v)                                                          \
  {                                                                            \
    std::cerr << #v << "\t:";                                                  \
    for (const auto &xxx : v) {                                                \
      std::cerr << xxx << " ";                                                 \
    }                                                                          \
    std::cerr << "\n";                                                         \
  }
#define SHOWVECTOR2(v)                                                         \
  {                                                                            \
    std::cerr << #v << "\t:\n";                                                \
    for (const auto &xxx : v) {                                                \
      for (const auto &yyy : xxx) {                                            \
        std::cerr << yyy << " ";                                               \
      }                                                                        \
      std::cerr << "\n";                                                       \
    }                                                                          \
  }
#define SHOWQUEUE(a)                                                           \
  {                                                                            \
    auto tmp(a);                                                               \
    std::cerr << #a << "\t:";                                                  \
    while (!tmp.empty()) {                                                     \
      std::cerr << tmp.front() << " ";                                         \
      tmp.pop();                                                               \
    }                                                                          \
    std::cerr << "\n";                                                         \
  }
// utility
#define ALL(a) (a).begin(), (a).end()
#define FOR(w, a, n) for (int w = (a); w < (n); ++w)
#define RFOR(w, a, n) for (int w = (n)-1; w >= (a); --w)
#define REP(w, n) for (int w = 0; w < int(n); ++w)
#define RREP(w, n) for (int w = int(n) - 1; w >= 0; --w)
#define IN(a, x, b) (a <= x && x < b)
template <class T> inline T CHMAX(T &a, const T b) {
  return a = (a < b) ? b : a;
}
template <class T> inline T CHMIN(T &a, const T b) {
  return a = (b < a) ? b : a;
}
#define EXCEPTION(msg)                                                         \
  throw std::string("Exception : " msg " [ in ") + __func__ + " : " +          \
      std::to_string(__LINE__) + " lines ]"
#define TRY(cond, msg)                                                         \
  try {                                                                        \
    if (cond)                                                                  \
      EXCEPTION(msg);                                                          \
  } catch (std::string s) {                                                    \
    std::cerr << s << std::endl;                                               \
  }
void CHECKTIME(std::function<void()> f) {
  auto start = std::chrono::system_clock::now();
  f();
  auto end = std::chrono::system_clock::now();
  auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start))
                 .count();
  std::cerr << res / (1.0e9) << "s\n";
}
// test
template <class T> using V = std::vector<T>;
template <class T> using VV = V<V<T>>;

template <typename S, typename T>
std::ostream &operator<<(std::ostream &os, std::pair<S, T> p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}

#define random_shuffle "USE std::shuffle!";

// type/const
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = (1 << 30) - 1;                   // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;             // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                         // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62); // 9.22x10^18
constexpr double EPS = 1e-10;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;
template <class T, size_t N> void FILL(T (&a)[N], const T &val) {
  for (auto &x : a)
    x = val;
}
template <class ARY, size_t N, size_t M, class T>
void FILL(ARY (&a)[N][M], const T &val) {
  for (auto &b : a)
    FILL(b, val);
}
template <class T> void FILL(std::vector<T> &a, const T &val) {
  for (auto &x : a)
    x = val;
}
template <class ARY, class T>
void FILL(std::vector<std::vector<ARY>> &a, const T &val) {
  for (auto &b : a)
    FILL(b, val);
}
// ------------>8------------------------------------->8------------

void calc(int n) {
  V<PAIR> e;
  REP(i, n) FOR(j, i + 1, n) e.emplace_back(i, j);

  std::map<int, int> map;
  REP(bit, 1 << e.size()) {
    VV<int> g(n, V<int>(n, INFINT));
    V<PAIR> ee;
    REP(i, e.size()) {
      if (bit & 1 << i) {
        g[e[i].first][e[i].second] = 1;
        g[e[i].second][e[i].first] = 1;
        ee.emplace_back(e[i]);
      }
    }

    REP(k, n) REP(i, n) REP(j, n) CHMIN(g[i][j], g[i][k] + g[k][j]);
    int cnt = 0;
    REP(i, n) FOR(j, i + 1, n) cnt += g[i][j] == 2;
    ++map[cnt];
  }
  SHOWVECTOR(map);
}

signed main() {
  INIT;

  VAR(int, n, k);
  if (k > (n - 1) * (n - 2) / 2) {
    OUT(-1) BR;
    return 0;
  }

  VV<bool> g(n, V<bool>(n, 0));
  V<PAIR> ans;
  FOR(i, 1, n) {
    ans.emplace_back(0, i);
    g[0][i] = g[i][0] = true;
  }

  int i = 1;
  REP(_, (n - 1) * (n - 2) / 2 - k) {
    [&] {
      while (true) {
        int j = i + 2;
        while (j < n && g[i][j])
          ++j;
        if (j >= n) {
          ++i;
          continue;
        }
        ans.emplace_back(i, j);
        g[i][j] = g[j][i] = true;
        return;
      }
    }();
  }

  OUT(ans.size()) BR;
  REP(i, ans.size()) { OUT(ans[i].first + 1) SP OUT(ans[i].second + 1) BR; }

  return 0;
}

Fixed submission:
// need
#include <algorithm>
#include <iostream>
// data structure
#include <bitset>
#include <complex>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
// #include <deque>
#include <array>
#include <unordered_map>
#include <unordered_set>
#include <valarray>
// stream
// #include <istream>
#include <sstream>
// #include <ostream>
#include <fstream>
// etc
#include <cassert>
#include <chrono>
#include <cmath>
#include <functional>
#include <iomanip>
#include <numeric>
#include <random>

// input
#define INIT                                                                   \
  std::ios::sync_with_stdio(false);                                            \
  std::cin.tie(0);
#define VAR(type, ...)                                                         \
  type __VA_ARGS__;                                                            \
  MACRO_VAR_Scan(__VA_ARGS__);
template <typename T> void MACRO_VAR_Scan(T &t) { std::cin >> t; }
template <typename First, typename... Rest>
void MACRO_VAR_Scan(First &first, Rest &...rest) {
  std::cin >> first;
  MACRO_VAR_Scan(rest...);
}
#define VEC_ROW(type, n, ...)                                                  \
  std::vector<type> __VA_ARGS__;                                               \
  MACRO_VEC_ROW_Init(n, __VA_ARGS__);                                          \
  for (int w_ = 0; w_ < n; ++w_) {                                             \
    MACRO_VEC_ROW_Scan(w_, __VA_ARGS__);                                       \
  }
template <typename T> void MACRO_VEC_ROW_Init(int n, T &t) { t.resize(n); }
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Init(int n, First &first, Rest &...rest) {
  first.resize(n);
  MACRO_VEC_ROW_Init(n, rest...);
}
template <typename T> void MACRO_VEC_ROW_Scan(int p, T &t) { std::cin >> t[p]; }
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Scan(int p, First &first, Rest &...rest) {
  std::cin >> first[p];
  MACRO_VEC_ROW_Scan(p, rest...);
}
#define VEC(type, c, n)                                                        \
  std::vector<type> c(n);                                                      \
  for (auto &i : c)                                                            \
    std::cin >> i;
#define MAT(type, c, m, n)                                                     \
  std::vector<std::vector<type>> c(m, std::vector<type>(n));                   \
  for (auto &R : c)                                                            \
    for (auto &w : R)                                                          \
      std::cin >> w;
// output
#define OUT(dist) std::cout << (dist);
#define FOUT(n, dist) std::cout << std::fixed << std::setprecision(n) << (dist);
#define SOUT(n, c, dist) std::cout << std::setw(n) << std::setfill(c) << (dist);
#define SP std::cout << " ";
#define TAB std::cout << "\t";
#define BR std::cout << "\n";
#define SPBR(w, n) std::cout << (w + 1 == n ? '\n' : ' ');
#define ENDL std::cout << std::endl;
#define FLUSH std::cout << std::flush;
#define SHOW(dist)                                                             \
  { std::cerr << #dist << "\t:" << (dist) << "\n"; }
#define SHOWVECTOR(v)                                                          \
  {                                                                            \
    std::cerr << #v << "\t:";                                                  \
    for (const auto &xxx : v) {                                                \
      std::cerr << xxx << " ";                                                 \
    }                                                                          \
    std::cerr << "\n";                                                         \
  }
#define SHOWVECTOR2(v)                                                         \
  {                                                                            \
    std::cerr << #v << "\t:\n";                                                \
    for (const auto &xxx : v) {                                                \
      for (const auto &yyy : xxx) {                                            \
        std::cerr << yyy << " ";                                               \
      }                                                                        \
      std::cerr << "\n";                                                       \
    }                                                                          \
  }
#define SHOWQUEUE(a)                                                           \
  {                                                                            \
    auto tmp(a);                                                               \
    std::cerr << #a << "\t:";                                                  \
    while (!tmp.empty()) {                                                     \
      std::cerr << tmp.front() << " ";                                         \
      tmp.pop();                                                               \
    }                                                                          \
    std::cerr << "\n";                                                         \
  }
// utility
#define ALL(a) (a).begin(), (a).end()
#define FOR(w, a, n) for (int w = (a); w < (n); ++w)
#define RFOR(w, a, n) for (int w = (n)-1; w >= (a); --w)
#define REP(w, n) for (int w = 0; w < int(n); ++w)
#define RREP(w, n) for (int w = int(n) - 1; w >= 0; --w)
#define IN(a, x, b) (a <= x && x < b)
template <class T> inline T CHMAX(T &a, const T b) {
  return a = (a < b) ? b : a;
}
template <class T> inline T CHMIN(T &a, const T b) {
  return a = (b < a) ? b : a;
}
#define EXCEPTION(msg)                                                         \
  throw std::string("Exception : " msg " [ in ") + __func__ + " : " +          \
      std::to_string(__LINE__) + " lines ]"
#define TRY(cond, msg)                                                         \
  try {                                                                        \
    if (cond)                                                                  \
      EXCEPTION(msg);                                                          \
  } catch (std::string s) {                                                    \
    std::cerr << s << std::endl;                                               \
  }
void CHECKTIME(std::function<void()> f) {
  auto start = std::chrono::system_clock::now();
  f();
  auto end = std::chrono::system_clock::now();
  auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start))
                 .count();
  std::cerr << res / (1.0e9) << "s\n";
}
// test
template <class T> using V = std::vector<T>;
template <class T> using VV = V<V<T>>;

template <typename S, typename T>
std::ostream &operator<<(std::ostream &os, std::pair<S, T> p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}

#define random_shuffle "USE std::shuffle!";

// type/const
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = (1 << 30) - 1;                   // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;             // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                         // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62); // 9.22x10^18
constexpr double EPS = 1e-10;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;
template <class T, size_t N> void FILL(T (&a)[N], const T &val) {
  for (auto &x : a)
    x = val;
}
template <class ARY, size_t N, size_t M, class T>
void FILL(ARY (&a)[N][M], const T &val) {
  for (auto &b : a)
    FILL(b, val);
}
template <class T> void FILL(std::vector<T> &a, const T &val) {
  for (auto &x : a)
    x = val;
}
template <class ARY, class T>
void FILL(std::vector<std::vector<ARY>> &a, const T &val) {
  for (auto &b : a)
    FILL(b, val);
}
// ------------>8------------------------------------->8------------

void calc(int n) {
  V<PAIR> e;
  REP(i, n) FOR(j, i + 1, n) e.emplace_back(i, j);

  std::map<int, int> map;
  REP(bit, 1 << e.size()) {
    VV<int> g(n, V<int>(n, INFINT));
    V<PAIR> ee;
    REP(i, e.size()) {
      if (bit & 1 << i) {
        g[e[i].first][e[i].second] = 1;
        g[e[i].second][e[i].first] = 1;
        ee.emplace_back(e[i]);
      }
    }

    REP(k, n) REP(i, n) REP(j, n) CHMIN(g[i][j], g[i][k] + g[k][j]);
    int cnt = 0;
    REP(i, n) FOR(j, i + 1, n) cnt += g[i][j] == 2;
    ++map[cnt];
  }
  SHOWVECTOR(map);
}

signed main() {
  INIT;

  VAR(int, n, k);
  if (k > (n - 1) * (n - 2) / 2) {
    OUT(-1) BR;
    return 0;
  }

  VV<bool> g(n, V<bool>(n, 0));
  V<PAIR> ans;
  FOR(i, 1, n) {
    ans.emplace_back(0, i);
    g[0][i] = g[i][0] = true;
  }

  int i = 1;
  REP(_, (n - 1) * (n - 2) / 2 - k) {
    [&] {
      while (true) {
        int j = i + 1;
        while (j < n && g[i][j])
          ++j;
        if (j >= n) {
          ++i;
          continue;
        }
        ans.emplace_back(i, j);
        g[i][j] = g[j][i] = true;
        return;
      }
    }();
  }

  OUT(ans.size()) BR;
  REP(i, ans.size()) { OUT(ans[i].first + 1) SP OUT(ans[i].second + 1) BR; }

  return 0;
}