Problem p02997 (C++)

Buggy submission (status: Time Limit Exceeded):
// include
//------------------------------------------
#include <bits/stdc++.h>
using namespace std;

// typedef
//------------------------------------------
typedef long long LL;
typedef vector<LL> VL;
typedef vector<VL> VVL;
typedef vector<string> VS;
typedef pair<LL, LL> PLL;

// container util
//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())

// constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int MOD = 1000000007;

// grid
//--------------------------------------------
VL dx = {0, 1, 0, -1};
VL dy = {1, 0, -1, 0};
VL dx2 = {-1, 0, 1, -1, 1, -1, 0, 1};
VL dy2 = {-1, -1, -1, 0, 0, 1, 1, 1};

// debug
//--------------------------------------------
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;

// IO accelerate
//--------------------------------------------
struct InitIO {
  InitIO() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(30);
  }
} init_io;

// template
//--------------------------------------------
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << (i + 1 == vec.size() ? "" : "\t");
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &vv) {
  for (int i = 0; i < vv.size(); ++i) {
    s << vv[i] << endl;
  }
  return s;
}

// main code

int main(int argc, char const *argv[]) {
  int n, k;
  cin >> n >> k;
  int n_max = (n - 1) * (n - 2) / 2;
  if (n_max < k) {
    cout << -1 << endl;
    return 0;
  }
  int m = n - 1 + (n_max - k);
  cout << m << endl;
  for (int i = 1; i <= n - 1; i++) {
    cout << 1 << " " << i + 1 << endl;
  }
  int count = 0;
  for (int i = 2;; i++) {
    for (int j = i + 1; j < n + 1; j++) {
      cout << i << " " << j << endl;
      count++;
      if (count == n_max - k) {
        return 0;
      }
    }
  }
  return 0;
}


Fixed submission:
// include
//------------------------------------------
#include <bits/stdc++.h>
using namespace std;

// typedef
//------------------------------------------
typedef long long LL;
typedef vector<LL> VL;
typedef vector<VL> VVL;
typedef vector<string> VS;
typedef pair<LL, LL> PLL;

// container util
//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())

// constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int MOD = 1000000007;

// grid
//--------------------------------------------
VL dx = {0, 1, 0, -1};
VL dy = {1, 0, -1, 0};
VL dx2 = {-1, 0, 1, -1, 1, -1, 0, 1};
VL dy2 = {-1, -1, -1, 0, 0, 1, 1, 1};

// debug
//--------------------------------------------
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;

// IO accelerate
//--------------------------------------------
struct InitIO {
  InitIO() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(30);
  }
} init_io;

// template
//--------------------------------------------
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << (i + 1 == vec.size() ? "" : "\t");
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &vv) {
  for (int i = 0; i < vv.size(); ++i) {
    s << vv[i] << endl;
  }
  return s;
}

// main code

int main(int argc, char const *argv[]) {
  int n, k;
  cin >> n >> k;
  int n_max = (n - 1) * (n - 2) / 2;
  if (n_max < k) {
    cout << -1 << endl;
    return 0;
  }
  int m = n - 1 + (n_max - k);
  cout << m << endl;
  for (int i = 1; i <= n - 1; i++) {
    cout << 1 << " " << i + 1 << endl;
  }
  if (n_max - k == 0) {
    return 0;
  }
  int count = 0;
  for (int i = 2;; i++) {
    for (int j = i + 1; j < n + 1; j++) {
      cout << i << " " << j << endl;
      count++;
      if (count == n_max - k) {
        return 0;
      }
    }
  }
  return 0;
}