Language: C++

#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, s, n) for (int i = s; i < (int)(n); i++)
#define rep3(i, s, n) for (int i = s; i > (int)(n); i--)
#define all(obj) obj.begin(), obj.end()
#define db(x) cerr << #x << ":" << x << " "
#define dbl(x) cerr << #x << ":" << x << "\n"
#define dbv(vec) cerr << #vec << ":"; for (auto e : vec) cerr << e << " "; cout << "\n"
#define dbvv(vv) cerr << #vv << ":\n"; for (auto vec : vv) { for (auto e : vec) cerr << e << " "; cerr << endl; }
#define YN(f) cout << (f ? "YES" : "NO") << endl
#define Yn(f) cout << (f ? "Yes" : "No") << endl
#define yn(f) cout << (f ? "yes" : "no") << endl
using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using pii = pair<int, int>;

int main() {
	int N; cin >> N;
	vector<pii> v;
	rep(i, N) {
		int P; cin >> P;
		pii p(P, i + 1); v.push_back(p);
	}
	sort(all(v), [](int x, int y) { return x > y; });
	int ans = 0;
	multiset<int> ms = { 0, 0, N + 1, N + 1};
	for (pii p : v) {
		int x = p.second;
		ms.insert(x);
		auto it = ms.lower_bound(x);
		auto l1 = next(it), l2 = next(l1);
		auto r1 = prev(it), r2 = prev(r1);
		ans += (*it - *l1) * (*r2 - *r1);
		ans += (*l1 - *l2) * (*r1 - *it);
	}
	cout << ans << endl;
	}

Compiler output:
In file included from /usr/include/c++/14/bits/stl_algobase.h:71,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Iter_comp_iter<_Compare>::operator()(_Iterator1, _Iterator2) [with _Iterator1 = __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int> > >; _Iterator2 = __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int> > >; _Compare = main()::<lambda(int, int)>]':
/usr/include/c++/14/bits/stl_algo.h:1777:14:   required from 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
 1777 |           if (__comp(__i, __first))
      |               ~~~~~~^~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1817:25:   required from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
 1817 |           std::__insertion_sort(__first, __first + int(_S_threshold), __comp);
      |           ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1908:31:   required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
 1908 |           std::__final_insertion_sort(__first, __last, __comp);
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4805:18:   required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = main()::<lambda(int, int)>]'
 4805 |       std::__sort(__first, __last, __gnu_cxx::__ops::__iter_comp_iter(__comp));
      |       ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:26:6:   required from here
   26 |         sort(all(v), [](int x, int y) { return x > y; });
      |         ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/predefined_ops.h:158:30: error: no match for call to '(main()::<lambda(int, int)>) (std::pair<int, int>&, std::pair<int, int>&)'
  158 |         { return bool(_M_comp(*__it1, *__it2)); }
      |                       ~~~~~~~^~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/predefined_ops.h:158:30: note: candidate: 'bool (*)(int, int)' (conversion)
/usr/include/c++/14/bits/predefined_ops.h:158:30: note:   candidate expects 3 arguments, 3 provided
a.cc:26:22: note: candidate: 'main()::<lambda(int, int)>'
   26 |         sort(all(v), [](int x, int y) { return x > y; });
      |                      ^
a.cc:26:22: note:   no known conversion for argument 1 from 'std::pair<int, int>' to 'int'
/usr/include/c++/14/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Val_comp_iter<_Compare>::operator()(_Value&, _Iterator) [with _Value = std::pair<int, int>; _Iterator = __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int> > >; _Compare = main()::<lambda(int, int)>]':
/usr/include/c++/14/bits/stl_algo.h:1757:20:   required from 'void std::__unguarded_linear_insert(_RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Val_comp_iter<main()::<lambda(int, int)> >]'
 1757 |       while (__comp(__val, __next))
      |              ~~~~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1785:36:   required from 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
 1785 |             std::__unguarded_linear_insert(__i,
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
 1786 |                                 __gnu_cxx::__ops::__val_comp_iter(__comp));
      |                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1817:25:   required from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
 1817 |           std::__insertion_sort(__first, __first + int(_S_threshold), __comp);
      |           ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1908:31:   required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
 1908 |           std::__final_insertion_sort(__first, __last, __comp);
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4805:18:   required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = main()::<lambda(int, int)>]'
 4805 |       std::__sort(__first, __last, __gnu_cxx::__ops::__iter_comp_iter(__comp));
      |       ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:26:6:   required from here
   26 |         sort(all(v), [](int x, int y) { return x > y; });
      |         ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/predefined_ops.h:240:30: error: no match for call to '(main()::<lambda(int, int)>) (std::pair<int, int>&, std::pair<int, int>&)'
  240 |         { return bool(_M_comp(__val, *__it)); }
      |                       ~~~~~~~^~~~~~~~~~~~~~
/usr/include/c++/14/bits/predefined_ops.h:240:30: note: candidate: 'bool (*)(int, int)' (conversion)
/usr/include/c++/14/bits/predefined_ops.h:240:30: note:   candidate expects 3 arguments, 3 provided
a.cc:26:22: note: candidate: 'main()::<lambda(int, int)>'
   26 |         sort(all(v), [](int x, int y) { return x > y; });
      |                      ^
a.cc:26:22: note:   no known conversion for argument 1 from 'std::pair<int, int>' to 'int'
/usr/include/c++/14/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Iter_comp_val<_Compare>::operator()(_Iterator, _Value&) [with _Iterator = __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int> > >; _Value = std::pair<int, int>; _Compare = main()::<lambda(int, int)>]':
/usr/include/c++/14/bits/stl_heap.h:140:48:   required from 'void std::__push_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Distance = long int; _Tp = pair<int, int>; _Compare = __gnu_cxx::__ops::_Iter_comp_val<main()::<lambda(int, int)> >]'
  140 |       while (__holeIndex > __topIndex && __comp(__first + __parent, __value))
      |                                          ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_heap.h:247:23:   required from 'void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Distance = long int; _Tp = pair<int, int>; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
  247 |       std::__push_heap(__first, __holeIndex, __topIndex,
      |       ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  248 |                        _GLIBCXX_MOVE(__value), __cmp);
      |                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_heap.h:356:22:   required from 'void std::__make_heap(_RandomAccessIterator, _RandomAccessIterator, _Compare&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
  356 |           std::__adjust_heap(__first, __parent, __len, _GLIBCXX_MOVE(__value),
      |           ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  357 |                              __comp);
      |                              ~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1593:23:   required from 'void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
 1593 |       std::__make_heap(__first, __middle, __comp);
      |       ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1868:25:   required from 'void std::__partial_sort(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
 1868 |       std::__heap_select(__first, __middle, __last, __comp);
      |       ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1884:27:   required from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pair<int, int>*, vector<pair<int, int> > >; _Size = long int; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(int, int)> >]'
 1884 |               std::__p