Language: C++

#include <bits/stdc++.h>

typedef long long int ll;
typedef unsigned long long int ull;
#define rep(i,n) for(auto i=0;i<(n);i++)
#define rep1(i,n) for(auto i=1;i<=(n);i++)
#define all(x) (x).begin(),(x).end()

using namespace std;


struct init{ init(){ cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(20); }; }aaaaaaa;


vector<vector<bool>> solve(int n){
    vector<vector<bool>> g(n, vector<int>(n, false));

    int d = 1+n;
    rep(src, n){
        rep(sink, n){
            if(src+1 + sink+1 != d){
                g[src][sink] = true;
            }
        }
    }

    return g;
}

int main(){
    int n;
    cin >> n;


    vector<vector<bool>> g;

    if(n%2 == 0){
        g = solve(n);
    }
    else{
        vector<vector<bool>> gg = solve(n-1);
        rep(i, n-1) gg[i].push_back(true);
        gg.push_back(vector<bool>(n, true));
        g = gg;
    }

    vector<pair<int, int>> res;
    rep(src, n){
        for(int sink = src+1; sink < n; sink++){
            if(g[src][sink]) res.push_back({src+1, sink+1});
        }
    }
    
    cout << res.size() << endl;
    for(auto p : res){
        cout << p.first << " " << p.second << endl;
    }
    
    return 0;
}

Compiler output:
a.cc: In function 'std::vector<std::vector<bool> > solve(int)':
a.cc:16:52: error: no matching function for call to 'std::vector<std::vector<bool> >::vector(int&, std::vector<int>)'
   16 |     vector<vector<bool>> g(n, vector<int>(n, false));
      |                                                    ^
In file included from /usr/include/c++/14/vector:66,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_vector.h:707:9: note: candidate: 'template<class _InputIterator, class> std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const allocator_type&) [with <template-parameter-2-2> = _InputIterator; _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >]'
  707 |         vector(_InputIterator __first, _InputIterator __last,
      |         ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:707:9: note:   template argument deduction/substitution failed:
a.cc:16:52: note:   deduced conflicting types for parameter '_InputIterator' ('int' and 'std::vector<int>')
   16 |     vector<vector<bool>> g(n, vector<int>(n, false));
      |                                                    ^
/usr/include/c++/14/bits/stl_vector.h:678:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >; allocator_type = std::allocator<std::vector<bool> >]'
  678 |       vector(initializer_list<value_type> __l,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:678:43: note:   no known conversion for argument 1 from 'int' to 'std::initializer_list<std::vector<bool> >'
  678 |       vector(initializer_list<value_type> __l,
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:659:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, std::__type_identity_t<_Alloc>&) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >; std::__type_identity_t<_Alloc> = std::allocator<std::vector<bool> >]'
  659 |       vector(vector&& __rv, const __type_identity_t<allocator_type>& __m)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:659:23: note:   no known conversion for argument 1 from 'int' to 'std::vector<std::vector<bool> >&&'
  659 |       vector(vector&& __rv, const __type_identity_t<allocator_type>& __m)
      |              ~~~~~~~~~^~~~
/usr/include/c++/14/bits/stl_vector.h:640:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, const allocator_type&, std::false_type) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >; allocator_type = std::allocator<std::vector<bool> >; std::false_type = std::false_type]'
  640 |       vector(vector&& __rv, const allocator_type& __m, false_type)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:640:7: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_vector.h:635:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, const allocator_type&, std::true_type) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >; allocator_type = std::allocator<std::vector<bool> >; std::true_type = std::true_type]'
  635 |       vector(vector&& __rv, const allocator_type& __m, true_type) noexcept
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:635:7: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_vector.h:624:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&, std::__type_identity_t<_Alloc>&) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >; std::__type_identity_t<_Alloc> = std::allocator<std::vector<bool> >]'
  624 |       vector(const vector& __x, const __type_identity_t<allocator_type>& __a)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:624:28: note:   no known conversion for argument 1 from 'int' to 'const std::vector<std::vector<bool> >&'
  624 |       vector(const vector& __x, const __type_identity_t<allocator_type>& __a)
      |              ~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:620:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >]'
  620 |       vector(vector&&) noexcept = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:620:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:601:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >]'
  601 |       vector(const vector& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:601:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:569:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(size_type, const value_type&, const allocator_type&) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >; size_type = long unsigned int; value_type = std::vector<bool>; allocator_type = std::allocator<std::vector<bool> >]'
  569 |       vector(size_type __n, const value_type& __value,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:569:47: note:   no known conversion for argument 2 from 'std::vector<int>' to 'const std::vector<std::vector<bool> >::value_type&' {aka 'const std::vector<bool>&'}
  569 |       vector(size_type __n, const value_type& __value,
      |                             ~~~~~~~~~~~~~~~~~~^~~~~~~
/usr/include/c++/14/bits/stl_vector.h:556:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(size_type, const allocator_type&) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >; size_type = long unsigned int; allocator_type = std::allocator<std::vector<bool> >]'
  556 |       vector(size_type __n, const allocator_type& __a = allocator_type())
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:556:51: note:   no known conversion for argument 2 from 'std::vector<int>' to 'const std::vector<std::vector<bool> >::allocator_type&' {aka 'const std::allocator<std::vector<bool> >&'}
  556 |       vector(size_type __n, const allocator_type& __a = allocator_type())
      |                             ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:542:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const allocator_type&) [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >; allocator_type = std::allocator<std::vector<bool> >]'
  542 |       vector(const allocator_type& __a) _GLIBCXX_NOEXCEPT
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:542:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:531:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector() [with _Tp = std::vector<bool>; _Alloc = std::allocator<std::vector<bool> >]'
  531 |       vector() = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:531:7: note:   candidate expects 0 arguments, 2 provided