Language: C++

#include<bits/stdc++.h>
using namespace std;
#define ll long long
typedef pair<int,int> P;
#define M 1000000007
#define all(a) (a).begin(),(a).end()
#define rep(i,n) reps(i,0,n)
#define reps(i,m,n) for(int i=(m);i<(n);i++)
int main(){
  int n,k;cin>>n>>k;
  if(k>(n-1)*(n-2)/2){
    cout<<-1;
    return 0;
  }
  vector<P> v;
  reps(i,1,n){
    v.push_back(i,n);
  }
  reps(i,1,(n-1)*(n-2)/2-k){
    if(i!=n-1) v.push_back(i,i+1);
    else v.push_back(1,i);
  }
  cout<<v.size()<<endl;
  rep(i,v.size()){
    cout<<v[i].first<<" "<<v[i].second <<endl;
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:17:16: error: no matching function for call to 'std::vector<std::pair<int, int> >::push_back(int&, int&)'
   17 |     v.push_back(i,n);
      |     ~~~~~~~~~~~^~~~~
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:1283:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::pair<int, int>; _Alloc = std::allocator<std::pair<int, int> >; value_type = std::pair<int, int>]'
 1283 |       push_back(const value_type& __x)
      |       ^~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1283:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:1300:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(value_type&&) [with _Tp = std::pair<int, int>; _Alloc = std::allocator<std::pair<int, int> >; value_type = std::pair<int, int>]'
 1300 |       push_back(value_type&& __x)
      |       ^~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1300:7: note:   candidate expects 1 argument, 2 provided
a.cc:20:27: error: no matching function for call to 'std::vector<std::pair<int, int> >::push_back(int&, int)'
   20 |     if(i!=n-1) v.push_back(i,i+1);
      |                ~~~~~~~~~~~^~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1283:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::pair<int, int>; _Alloc = std::allocator<std::pair<int, int> >; value_type = std::pair<int, int>]'
 1283 |       push_back(const value_type& __x)
      |       ^~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1283:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:1300:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(value_type&&) [with _Tp = std::pair<int, int>; _Alloc = std::allocator<std::pair<int, int> >; value_type = std::pair<int, int>]'
 1300 |       push_back(value_type&& __x)
      |       ^~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1300:7: note:   candidate expects 1 argument, 2 provided
a.cc:21:21: error: no matching function for call to 'std::vector<std::pair<int, int> >::push_back(int, int&)'
   21 |     else v.push_back(1,i);
      |          ~~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/stl_vector.h:1283:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::pair<int, int>; _Alloc = std::allocator<std::pair<int, int> >; value_type = std::pair<int, int>]'
 1283 |       push_back(const value_type& __x)
      |       ^~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1283:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:1300:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(value_type&&) [with _Tp = std::pair<int, int>; _Alloc = std::allocator<std::pair<int, int> >; value_type = std::pair<int, int>]'
 1300 |       push_back(value_type&& __x)
      |       ^~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1300:7: note:   candidate expects 1 argument, 2 provided