In file included from /usr/include/c++/11/map:60,
                 from /usr/include/x86_64-linux-gnu/c++/11/bits/stdc++.h:81,
                 from answer.code:1:
/usr/include/c++/11/bits/stl_tree.h: In instantiation of ‘static const _Key& std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_S_key(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Const_Link_type) [with _Key = int; _Val = int; _KeyOfValue = std::_Identity<int>; _Compare = cmp; _Alloc = std::allocator<int>; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Const_Link_type = const std::_Rb_tree_node<int>*]’:
/usr/include/c++/11/bits/stl_tree.h:2069:47:   required from ‘std::pair<std::_Rb_tree_node_base*, std::_Rb_tree_node_base*> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_get_insert_unique_pos(const key_type&) [with _Key = int; _Val = int; _KeyOfValue = std::_Identity<int>; _Compare = cmp; _Alloc = std::allocator<int>; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::key_type = int]’
/usr/include/c++/11/bits/stl_tree.h:2122:4:   required from ‘std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(_Arg&&) [with _Arg = const int&; _Key = int; _Val = int; _KeyOfValue = std::_Identity<int>; _Compare = cmp; _Alloc = std::allocator<int>]’
/usr/include/c++/11/bits/stl_set.h:512:25:   required from ‘std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(const value_type&) [with _Key = int; _Compare = cmp; _Alloc = std::allocator<int>; typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator = std::_Rb_tree<int, int, std::_Identity<int>, cmp, std::allocator<int> >::const_iterator; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other = std::allocator<int>; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key> = __gnu_cxx::__alloc_traits<std::allocator<int>, int>::rebind<int>; typename _Alloc::value_type = int; std::set<_Key, _Compare, _Alloc>::value_type = int]’
answer.code:45:25:   required from here
/usr/include/c++/11/bits/stl_tree.h:770:15: error: static assertion failed: comparison object must be invocable as const
  770 |               is_invocable_v<const _Compare&, const _Key&, const _Key&>,
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/11/bits/stl_tree.h:770:15: note: ‘std::is_invocable_v<const cmp&, const int&, const int&>’ evaluates to false