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

#pragma GCC optimize("O3")
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <iomanip>
#include <iostream>
#include <istream>
#include <iterator>
#include <list>
#include <map>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
#define IINF 100000000
#define INF 300000000000000000
#define MOD 1000
#define mod 1000000007
#define INT_MAX_ 2147483647
#define REP(i, a, n) for (ll i = a; i < (ll)(n); i++)
#define REPE(i, a, n) for (ll i = a; i <= (ll)(n); i++)
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define Endl endl
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define eb emplace_back
#define mmax(x, y) (x > y ? x : y)
#define mmin(x, y) (x < y ? x : y)
#define chmax(x, y) x = mmax(x, y)
#define chmin(x, y) x = mmin(x, y)
#define all(x) (x).begin(), (x).end()
#define siz(x) (ll)(x).size()
#define PI acos(-1.0)
#define me memset
using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;
typedef long double ld;
typedef pair<int, int> Pin;
typedef pair<ll, ll> Pll;
template <class T> using V = vector<T>;
template <typename T>
using min_priority_queue = priority_queue<T, vector<T>, greater<T>>;
long long GCD(long long a, long long b) { return b ? GCD(b, a % b) : a; }
long long LCM(long long a, long long b) { return a / GCD(a, b) * b; }
int dx[5] = {-1, 0, 1, 0, 0};
int dy[5] = {0, -1, 0, 1, 0};
int ddx[8] = {-1, 0, 1, 0, 1, 1, -1, -1};
int ddy[8] = {0, -1, 0, 1, 1, -1, 1, -1};
ll cmp1(pair<ll, ll> a, pair<ll, ll> b) {
  if (a.se != b.se)
    return a.se < b.se;
  else
    return a.fi < b.fi;
}
//----------------------------------------------------------------------

//----------------------------------------------------------------------
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  //-------------------------------
  // ll begin_time=clock();
  //-------------------------------
  ll n, m;
  cin >> n >> m;
  V<Pll> ans;
  ll cnt = m;
  if (n % 2 == 0) {
    // even
    ll l = n / 2;
    ll s = 0;
    for (ll i = 0; i < m; i++) {
      if ((l + 1 + i) - (l - i) >= l)
        s = 1;
      ans.pb(mp(l - i - s, l + 1 + i));
    }
  } else {
    for (ll i = n / 2; i >= 1; i--) {
      ans.pb(mp(i, n - i));
      cnt--;
      if (cnt == 0)
        break;
    }
  }
  for (ll i = 0; i < m; i++) {
    cout << ans[i].fi << " " << ans[i].se << endl;
  }
  //-------------------------------
  // ll end_time=clock();cout<<"time="<<end_time-begin_time<<"ms"<<endl;
  //-------------------------------
  return 0;
}
//----------------------------------------------------------------------


Error: WA

Fixed submission:

#pragma GCC optimize("O3")
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <iomanip>
#include <iostream>
#include <istream>
#include <iterator>
#include <list>
#include <map>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
#define IINF 100000000
#define INF 300000000000000000
#define MOD 1000
#define mod 1000000007
#define INT_MAX_ 2147483647
#define REP(i, a, n) for (ll i = a; i < (ll)(n); i++)
#define REPE(i, a, n) for (ll i = a; i <= (ll)(n); i++)
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define Endl endl
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define eb emplace_back
#define mmax(x, y) (x > y ? x : y)
#define mmin(x, y) (x < y ? x : y)
#define chmax(x, y) x = mmax(x, y)
#define chmin(x, y) x = mmin(x, y)
#define all(x) (x).begin(), (x).end()
#define siz(x) (ll)(x).size()
#define PI acos(-1.0)
#define me memset
using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;
typedef long double ld;
typedef pair<int, int> Pin;
typedef pair<ll, ll> Pll;
template <class T> using V = vector<T>;
template <typename T>
using min_priority_queue = priority_queue<T, vector<T>, greater<T>>;
long long GCD(long long a, long long b) { return b ? GCD(b, a % b) : a; }
long long LCM(long long a, long long b) { return a / GCD(a, b) * b; }
int dx[5] = {-1, 0, 1, 0, 0};
int dy[5] = {0, -1, 0, 1, 0};
int ddx[8] = {-1, 0, 1, 0, 1, 1, -1, -1};
int ddy[8] = {0, -1, 0, 1, 1, -1, 1, -1};
ll cmp1(pair<ll, ll> a, pair<ll, ll> b) {
  if (a.se != b.se)
    return a.se < b.se;
  else
    return a.fi < b.fi;
}
//----------------------------------------------------------------------

//----------------------------------------------------------------------
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  //-------------------------------
  // ll begin_time=clock();
  //-------------------------------
  ll n, m;
  cin >> n >> m;
  V<Pll> ans;
  ll cnt = m;
  if (n % 2 == 0) {
    for (ll i = 1; i <= n; i += 3) {
      if (i + 2 <= n)
        ans.pb(mp(i, i + 2));
      else
        ans.pb(mp(i, i + 1));
      cnt--;
      if (cnt == 0)
        break;
    }
  } else {
    for (ll i = n / 2; i >= 1; i--) {
      ans.pb(mp(i, n - i));
      cnt--;
      if (cnt == 0)
        break;
    }
  }
  for (ll i = m - 1; i >= 0; i--) {
    cout << ans[i].fi << " " << ans[i].se << endl;
  }
  //-------------------------------
  // ll end_time=clock();cout<<"time="<<end_time-begin_time<<"ms"<<endl;
  //-------------------------------
  return 0;
}
//----------------------------------------------------------------------