Problem p02919 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, s, N) for (ll i{s}; i < (N); i++)
#define rem(i, N, s) for (ll i{N}; i > (s); i--)
#define debug(x) cerr << #x << ": " << x << '\n'
#define debugp(i) cerr << "(" << i.first << ", " << i.second << ")\n"
#define debugv(v)                                                              \
  cerr << #v << ": ";                                                          \
  for (auto &&i : v)                                                           \
    cerr << i << " ";                                                          \
  cerr << '\n'
#define debugvp(v)                                                             \
  cerr << #v << ": ";                                                          \
  for (auto &&i : v)                                                           \
    cerr << "(" << i.first << ", " << i.second << ") ";                        \
  cerr << '\n'
const int MOD = (int)1e9 + 7;
const long double pi = 3.141592653589793238462643383279L;
using namespace std;
using ll = long long;
using ld = long double;
using str = string;
using wstr = wstring;
const string rt = "\n", sp = " ";
const wstring wrt = L"\n", wsp = L" ";

/* GCD */
template <typename M, typename N> constexpr common_type_t<M, N> gcd(M a, N b) {
  return b != 0 ? gcd(b, a % b) : a;
}

/* LCM */
template <typename M, typename N> constexpr common_type_t<M, N> lcm(M a, N b) {
  return a * b / gcd(a, b);
}

/* UNIONFIND */
template <typename T> struct UnionFind {
  vector<T> par;
  UnionFind(T n) : par(n, -1) {}
  void init(T n) { par.assign(n, -1); }
  T root(T x) {
    if (par[x] < 0)
      return x;
    else
      return par[x] = root(par[x]);
  }
  bool issame(T x, T y) { return root(x) == root(y); }
  bool merge(T x, T y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (par[x] > par[y])
      swap(x, y);
    par[x] += par[y];
    par[y] = x;
    return true;
  }
  int size(int x) { return -par[root(x)]; }
};

/* COMB */
auto comb(ll N) {
  vector<vector<ll>> v(N + 1, vector<ll>(N + 1));
  for (unsigned int i = 0; i < v.size(); i++) {
    v[i][0] = 1;
    v[i][i] = 1;
  }
  for (unsigned int k = 1; k < v.size(); k++) {
    for (unsigned int j = 1; j < k; j++) {
      v[k][j] = (v[k - 1][j - 1] + v[k - 1][j]);
    }
  }
  return v;
}
auto comb(ll N, ll n) {
  vector<vector<ll>> v(max(N + 1, n + 1), vector<ll>(max(N + 1, n + 1)));
  for (unsigned int i = 0; i < v.size(); i++) {
    v[i][0] = 1;
    v[i][i] = 1;
  }
  for (unsigned int k = 1; k < v.size(); k++) {
    for (unsigned int j = 1; j < k; j++) {
      v[k][j] = (v[k - 1][j - 1] + v[k - 1][j]);
    }
  }
  return v[N][n];
}

/* COMB % MOD */
template <typename T> ll combpm(T N_, T C_) {
  const int NUM_ = 400001;
  static ll fact[NUM_ + 1], factr[NUM_ + 1], inv[NUM_ + 1];
  if (fact[0] == 0) {
    inv[1] = fact[0] = factr[0] = 1;
    for (int i = 2; i <= NUM_; ++i)
      inv[i] = inv[MOD % i] * (MOD - MOD / i) % MOD;
    for (int i = 1; i <= NUM_; ++i)
      fact[i] = fact[i - 1] * i % MOD, factr[i] = factr[i - 1] * inv[i] % MOD;
  }
  if (C_ < 0 || C_ > N_)
    return 0;
  return factr[C_] * fact[N_] % MOD * factr[N_ - C_] % MOD;
}

/* MAKE VECTOR */
template <class T> constexpr vector<T> make_vector(size_t a) {
  return vector<T>(a);
}
template <class T, class... Ts> auto make_vector(size_t a, Ts... ts) {
  return vector<decltype(make_vector<T>(ts...))>(a, make_vector<T>(ts...));
}

/* MAKE DEQUE */
template <class T> constexpr deque<T> make_deque(size_t a) {
  return deque<T>(a);
}
template <class T, class... Ts> auto make_deque(size_t a, Ts... ts) {
  return deque<decltype(make_deque<T>(ts...))>(a, make_deque<T>(ts...));
}

/* TEST */
void test(ll n) { cout << "test" << n << endl; }

/* PRECISION */
void fixsp(ll n) { cout << fixed << setprecision(n); }
void defsp(ll n) { cout << defaultfloat << setprecision(n); }

/* WEIGHTENED UNIONFIND */
struct WUnionFind {
  vector<int> par;
  vector<int> rank;
  WUnionFind(int n = 1) { init(n); }
  void init(int n = 1) {
    par.resize(n);
    rank.resize(n);
    for (int i = 0; i < n; ++i)
      par[i] = i, rank[i] = 0;
  }
  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }
  bool issame(int x, int y) { return root(x) == root(y); }
  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    return true;
  }
};

/* DIVISOR */
deque<ll> divisor(ll n) {
  deque<ll> ret;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n)
        ret.push_back(n / i);
    }
  }
  sort(begin(ret), end(ret));
  return (ret);
}

/* MODE */
template <typename T> auto mode(T data) {
  if (data.size() == 1)
    return *begin(data);
  sort(begin(data), end(data));
  typename decltype(data)::value_type mode{};
  size_t n{}, count{1};
  for (auto iter = adjacent_find(begin(data), end(data)), last = end(data),
            next = end(data);
       iter != last;) {
    next = upper_bound(iter, last, *iter);
    count = distance(iter, next);
    if (n < count)
      n = count, mode = *iter;
    iter = adjacent_find(next, last);
  }
  return mode;
}

/* MEDIAN */
template <typename T> auto median(T data) {
  sort(begin(data), end(data));
  size_t median_index = data.size() / 2;
  return (
      data.size() % 2 == 0
          ? static_cast<double>(data[median_index] + data[median_index - 1]) / 2
          : data[median_index]);
}

/* INT POW */
template <typename T> T multi(T a, ll b) {
  T ans{1};
  for (int i{}; i < b; i++)
    ans *= a;
  return ans;
}

/* INF */
template <typename T> constexpr T inf() { return numeric_limits<T>::max(); }

/* FASTER IO */
void fastio(ll a) {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  cerr.tie(nullptr);
  if (a)
    fixsp(a);
}

/* MIN COST FLOW */
template <typename flow_t, typename cost_t> struct PrimalDual {
  const cost_t INF;
  struct edge {
    int to;
    flow_t cap;
    cost_t cost;
    int rev;
    bool isrev;
  };
  vector<vector<edge>> graph;
  vector<cost_t> potential, min_cost;
  vector<int> prevv, preve;
  PrimalDual(int V) : graph(V), INF(numeric_limits<cost_t>::max()) {}
  void add_edge(int from, int to, flow_t cap, cost_t cost) {
    graph[from].emplace_back(
        (edge){to, cap, cost, (int)graph[to].size(), false});
    graph[to].emplace_back(
        (edge){from, 0, -cost, (int)graph[from].size() - 1, true});
  }
  cost_t min_cost_flow(int s, int t, flow_t f) {
    int V = (int)graph.size();
    cost_t ret = 0;
    using Pi = pair<cost_t, int>;
    priority_queue<Pi, vector<Pi>, greater<Pi>> que;
    potential.assign(V, 0);
    preve.assign(V, -1);
    prevv.assign(V, -1);
    while (f > 0) {
      min_cost.assign(V, INF);
      que.emplace(0, s);
      min_cost[s] = 0;
      while (!que.empty()) {
        Pi p = que.top();
        que.pop();
        if (min_cost[p.second] < p.first)
          continue;
        for (int i = 0; i < graph[p.second].size(); i++) {
          edge &e = graph[p.second][i];
          cost_t nextCost = min_cost[p.second] + e.cost + potential[p.second] -
                            potential[e.to];
          if (e.cap > 0 && min_cost[e.to] > nextCost) {
            min_cost[e.to] = nextCost;
            prevv[e.to] = p.second, preve[e.to] = i;
            que.emplace(min_cost[e.to], e.to);
          }
        }
      }
      if (min_cost[t] == INF)
        return -1;
      for (int v = 0; v < V; v++)
        potential[v] += min_cost[v];
      flow_t addflow = f;
      for (int v = t; v != s; v = prevv[v]) {
        addflow = min(addflow, graph[prevv[v]][preve[v]].cap);
      }
      f -= addflow;
      ret += addflow * potential[t];
      for (int v = t; v != s; v = prevv[v]) {
        edge &e = graph[prevv[v]][preve[v]];
        e.cap -= addflow;
        graph[v][e.rev].cap += addflow;
      }
    }
    return ret;
  }
  void output() {
    for (int i = 0; i < graph.size(); i++) {
      for (auto &e : graph[i]) {
        if (e.isrev)
          continue;
        auto &rev_e = graph[e.to][e.rev];
        cout << i << "->" << e.to << " (flow: " << rev_e.cap << "/"
             << rev_e.cap + e.cap << ")" << endl;
      }
    }
  }
};

/* BELLMANFORD */

vector<long long>
BellmanFord(int s, int V, vector<tuple<long long, long long, long long>> &G) {
  vector<long long> d(V, inf<long long>());
  vector<bool> neg(V);
  d[s] = 0;
  for (int i = 0; i < V - 1; ++i) {
    for (auto &&e : G) {
      if (d[get<0>(e)] == inf<long long>())
        continue;
      d[get<1>(e)] = min(d[get<1>(e)], d[get<0>(e)] + get<2>(e));
    }
  }
  for (int i = 0; i < V; ++i) {
    for (auto &&e : G) {
      if (d[get<0>(e)] == inf<long long>())
        continue;
      if (d[get<1>(e)] > d[get<0>(e)] + get<2>(e)) {
        d[get<1>(e)] = d[get<0>(e)] + get<2>(e);
        neg[get<1>(e)] = 1;
      }
      if (neg[get<0>(e)])
        neg[get<1>(e)] = 1;
    }
  }
  for (int i = 0; i < V; i++) {
    if (neg[i])
      d[i] = -inf<long long>();
  }
  return d;
}

/* DIJKSTRA */

vector<long long> Dijkstra(int s, int V,
                           vector<tuple<long long, long long, long long>> &G) {
  vector<vector<pair<long long, long long>>> uv(V);
  for (auto &&i : G)
    uv[get<0>(i)].emplace_back(get<1>(i), get<2>(i));
  vector<long long> d(V, inf<long long>() / 2);
  vector<long long> prev(V, -1);
  d[s] = 0;
  priority_queue<pair<long long, long long>, vector<pair<long long, long long>>,
                 greater<pair<long long, long long>>>
      Q;
  rep(i, 0, V) Q.emplace(d[i], i);
  while (!Q.empty()) {
    long long u = Q.top().second;
    Q.pop();
    for (auto &&v : uv[u]) {
      long long alt = d[u] + v.second;
      if (d[v.first] > alt) {
        d[v.first] = alt;
        if (d[v.first] > inf<long long>() / 4)
          d[v.first] = inf<long long>() / 2;
        else if (d[v.first] < -inf<long long>() / 4)
          d[v.first] = -inf<long long>() / 2;
        prev[v.first] = u;
        Q.emplace(alt, v.first);
      }
    }
  }
  for (auto &&i : d) {
    if (abs(i) > inf<long long>() / 4)
      (i *= 2)++;
  }
  return d;
}

/* BFS */

vector<long long> BFS(long long s, long long V,
                      vector<pair<long long, long long>> &G) {
  queue<long long> Q;
  vector<vector<long long>> uv(V);
  for (auto &&i : G)
    uv[i.first].emplace_back(i.second);
  vector<long long> d(V, inf<long long>());
  d[s] = 0;
  Q.emplace(s);
  while (!Q.empty()) {
    long long v = Q.front();
    Q.pop();
    for (auto &&i : uv[v]) {
      if (d[i] == inf<long long>()) {
        d[i] = d[v] + 1;
        Q.emplace(i);
      }
    }
  }
  return d;
}

/* WARSHALLFLOYD */
vector<vector<long long>>
WarshallFloyd(int V, vector<tuple<long long, long long, long long>> &G) {
  vector<vector<long long>> d(V, vector<long long>(V, inf<long long>()));
  for (auto &&i : G) {
    d[get<0>(i)][get<1>(i)] = get<2>(i);
  }
  rep(k, 0, V) rep(i, 0, V) rep(j, 0, V) {
    if (d[i][k] != inf<ll>() && d[k][j] != inf<ll>())
      d[i][j] = min(d[i][j], d[i][k] + d[k][j]);
  }
  rep(i, 0, V) { d[i][i] = 0; }
  return d;
}

/* NUITA.INC & KYOTO UNIVERSITY */

/* LAST EDITED ON 9.5.2019 */

/* CODE STARTS FROM HERE */

int main() {
  fastio(0);
  ll N;
  cin >> N;
  auto P = make_deque<ll>(0);
  rep(i, 0, N) {
    ll p;
    cin >> p;
    P.emplace_back(p);
  }
  auto dir = make_deque<ll>(N);
  rep(i, 0, N) { dir[P[i] - 1] = i; }
  multiset<ll> se;
  rep(i, 0, 2) se.insert(-1);
  rep(i, 0, 2) se.insert(N);
  ll ans{};
  rem(i, N, 0) {
    se.insert(dir[i - 1]);
    auto it = lower_bound(all(se), dir[i - 1]);
    it--;
    it--;
    ll l2 = *it;
    it++;
    ll l1 = *it;
    it++;
    ll c = *it;
    it++;
    ll r1 = *it;
    it++;
    ll r2 = *it;
    ans += i * ((l1 - l2) * (r1 - c) + (c - l1) * (r2 - r1));
  }
  cout << ans;
}

Fixed submission:
#include <bits/stdc++.h>
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, s, N) for (ll i{s}; i < (N); i++)
#define rem(i, N, s) for (ll i{N}; i > (s); i--)
#define debug(x) cerr << #x << ": " << x << '\n'
#define debugp(i) cerr << "(" << i.first << ", " << i.second << ")\n"
#define debugv(v)                                                              \
  cerr << #v << ": ";                                                          \
  for (auto &&i : v)                                                           \
    cerr << i << " ";                                                          \
  cerr << '\n'
#define debugvp(v)                                                             \
  cerr << #v << ": ";                                                          \
  for (auto &&i : v)                                                           \
    cerr << "(" << i.first << ", " << i.second << ") ";                        \
  cerr << '\n'
const int MOD = (int)1e9 + 7;
const long double pi = 3.141592653589793238462643383279L;
using namespace std;
using ll = long long;
using ld = long double;
using str = string;
using wstr = wstring;
const string rt = "\n", sp = " ";
const wstring wrt = L"\n", wsp = L" ";

/* GCD */
template <typename M, typename N> constexpr common_type_t<M, N> gcd(M a, N b) {
  return b != 0 ? gcd(b, a % b) : a;
}

/* LCM */
template <typename M, typename N> constexpr common_type_t<M, N> lcm(M a, N b) {
  return a * b / gcd(a, b);
}

/* UNIONFIND */
template <typename T> struct UnionFind {
  vector<T> par;
  UnionFind(T n) : par(n, -1) {}
  void init(T n) { par.assign(n, -1); }
  T root(T x) {
    if (par[x] < 0)
      return x;
    else
      return par[x] = root(par[x]);
  }
  bool issame(T x, T y) { return root(x) == root(y); }
  bool merge(T x, T y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (par[x] > par[y])
      swap(x, y);
    par[x] += par[y];
    par[y] = x;
    return true;
  }
  int size(int x) { return -par[root(x)]; }
};

/* COMB */
auto comb(ll N) {
  vector<vector<ll>> v(N + 1, vector<ll>(N + 1));
  for (unsigned int i = 0; i < v.size(); i++) {
    v[i][0] = 1;
    v[i][i] = 1;
  }
  for (unsigned int k = 1; k < v.size(); k++) {
    for (unsigned int j = 1; j < k; j++) {
      v[k][j] = (v[k - 1][j - 1] + v[k - 1][j]);
    }
  }
  return v;
}
auto comb(ll N, ll n) {
  vector<vector<ll>> v(max(N + 1, n + 1), vector<ll>(max(N + 1, n + 1)));
  for (unsigned int i = 0; i < v.size(); i++) {
    v[i][0] = 1;
    v[i][i] = 1;
  }
  for (unsigned int k = 1; k < v.size(); k++) {
    for (unsigned int j = 1; j < k; j++) {
      v[k][j] = (v[k - 1][j - 1] + v[k - 1][j]);
    }
  }
  return v[N][n];
}

/* COMB % MOD */
template <typename T> ll combpm(T N_, T C_) {
  const int NUM_ = 400001;
  static ll fact[NUM_ + 1], factr[NUM_ + 1], inv[NUM_ + 1];
  if (fact[0] == 0) {
    inv[1] = fact[0] = factr[0] = 1;
    for (int i = 2; i <= NUM_; ++i)
      inv[i] = inv[MOD % i] * (MOD - MOD / i) % MOD;
    for (int i = 1; i <= NUM_; ++i)
      fact[i] = fact[i - 1] * i % MOD, factr[i] = factr[i - 1] * inv[i] % MOD;
  }
  if (C_ < 0 || C_ > N_)
    return 0;
  return factr[C_] * fact[N_] % MOD * factr[N_ - C_] % MOD;
}

/* MAKE VECTOR */
template <class T> constexpr vector<T> make_vector(size_t a) {
  return vector<T>(a);
}
template <class T, class... Ts> auto make_vector(size_t a, Ts... ts) {
  return vector<decltype(make_vector<T>(ts...))>(a, make_vector<T>(ts...));
}

/* MAKE DEQUE */
template <class T> constexpr deque<T> make_deque(size_t a) {
  return deque<T>(a);
}
template <class T, class... Ts> auto make_deque(size_t a, Ts... ts) {
  return deque<decltype(make_deque<T>(ts...))>(a, make_deque<T>(ts...));
}

/* TEST */
void test(ll n) { cout << "test" << n << endl; }

/* PRECISION */
void fixsp(ll n) { cout << fixed << setprecision(n); }
void defsp(ll n) { cout << defaultfloat << setprecision(n); }

/* WEIGHTENED UNIONFIND */
struct WUnionFind {
  vector<int> par;
  vector<int> rank;
  WUnionFind(int n = 1) { init(n); }
  void init(int n = 1) {
    par.resize(n);
    rank.resize(n);
    for (int i = 0; i < n; ++i)
      par[i] = i, rank[i] = 0;
  }
  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }
  bool issame(int x, int y) { return root(x) == root(y); }
  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    return true;
  }
};

/* DIVISOR */
deque<ll> divisor(ll n) {
  deque<ll> ret;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n)
        ret.push_back(n / i);
    }
  }
  sort(begin(ret), end(ret));
  return (ret);
}

/* MODE */
template <typename T> auto mode(T data) {
  if (data.size() == 1)
    return *begin(data);
  sort(begin(data), end(data));
  typename decltype(data)::value_type mode{};
  size_t n{}, count{1};
  for (auto iter = adjacent_find(begin(data), end(data)), last = end(data),
            next = end(data);
       iter != last;) {
    next = upper_bound(iter, last, *iter);
    count = distance(iter, next);
    if (n < count)
      n = count, mode = *iter;
    iter = adjacent_find(next, last);
  }
  return mode;
}

/* MEDIAN */
template <typename T> auto median(T data) {
  sort(begin(data), end(data));
  size_t median_index = data.size() / 2;
  return (
      data.size() % 2 == 0
          ? static_cast<double>(data[median_index] + data[median_index - 1]) / 2
          : data[median_index]);
}

/* INT POW */
template <typename T> T multi(T a, ll b) {
  T ans{1};
  for (int i{}; i < b; i++)
    ans *= a;
  return ans;
}

/* INF */
template <typename T> constexpr T inf() { return numeric_limits<T>::max(); }

/* FASTER IO */
void fastio(ll a) {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  cerr.tie(nullptr);
  if (a)
    fixsp(a);
}

/* MIN COST FLOW */
template <typename flow_t, typename cost_t> struct PrimalDual {
  const cost_t INF;
  struct edge {
    int to;
    flow_t cap;
    cost_t cost;
    int rev;
    bool isrev;
  };
  vector<vector<edge>> graph;
  vector<cost_t> potential, min_cost;
  vector<int> prevv, preve;
  PrimalDual(int V) : graph(V), INF(numeric_limits<cost_t>::max()) {}
  void add_edge(int from, int to, flow_t cap, cost_t cost) {
    graph[from].emplace_back(
        (edge){to, cap, cost, (int)graph[to].size(), false});
    graph[to].emplace_back(
        (edge){from, 0, -cost, (int)graph[from].size() - 1, true});
  }
  cost_t min_cost_flow(int s, int t, flow_t f) {
    int V = (int)graph.size();
    cost_t ret = 0;
    using Pi = pair<cost_t, int>;
    priority_queue<Pi, vector<Pi>, greater<Pi>> que;
    potential.assign(V, 0);
    preve.assign(V, -1);
    prevv.assign(V, -1);
    while (f > 0) {
      min_cost.assign(V, INF);
      que.emplace(0, s);
      min_cost[s] = 0;
      while (!que.empty()) {
        Pi p = que.top();
        que.pop();
        if (min_cost[p.second] < p.first)
          continue;
        for (int i = 0; i < graph[p.second].size(); i++) {
          edge &e = graph[p.second][i];
          cost_t nextCost = min_cost[p.second] + e.cost + potential[p.second] -
                            potential[e.to];
          if (e.cap > 0 && min_cost[e.to] > nextCost) {
            min_cost[e.to] = nextCost;
            prevv[e.to] = p.second, preve[e.to] = i;
            que.emplace(min_cost[e.to], e.to);
          }
        }
      }
      if (min_cost[t] == INF)
        return -1;
      for (int v = 0; v < V; v++)
        potential[v] += min_cost[v];
      flow_t addflow = f;
      for (int v = t; v != s; v = prevv[v]) {
        addflow = min(addflow, graph[prevv[v]][preve[v]].cap);
      }
      f -= addflow;
      ret += addflow * potential[t];
      for (int v = t; v != s; v = prevv[v]) {
        edge &e = graph[prevv[v]][preve[v]];
        e.cap -= addflow;
        graph[v][e.rev].cap += addflow;
      }
    }
    return ret;
  }
  void output() {
    for (int i = 0; i < graph.size(); i++) {
      for (auto &e : graph[i]) {
        if (e.isrev)
          continue;
        auto &rev_e = graph[e.to][e.rev];
        cout << i << "->" << e.to << " (flow: " << rev_e.cap << "/"
             << rev_e.cap + e.cap << ")" << endl;
      }
    }
  }
};

/* BELLMANFORD */

vector<long long>
BellmanFord(int s, int V, vector<tuple<long long, long long, long long>> &G) {
  vector<long long> d(V, inf<long long>());
  vector<bool> neg(V);
  d[s] = 0;
  for (int i = 0; i < V - 1; ++i) {
    for (auto &&e : G) {
      if (d[get<0>(e)] == inf<long long>())
        continue;
      d[get<1>(e)] = min(d[get<1>(e)], d[get<0>(e)] + get<2>(e));
    }
  }
  for (int i = 0; i < V; ++i) {
    for (auto &&e : G) {
      if (d[get<0>(e)] == inf<long long>())
        continue;
      if (d[get<1>(e)] > d[get<0>(e)] + get<2>(e)) {
        d[get<1>(e)] = d[get<0>(e)] + get<2>(e);
        neg[get<1>(e)] = 1;
      }
      if (neg[get<0>(e)])
        neg[get<1>(e)] = 1;
    }
  }
  for (int i = 0; i < V; i++) {
    if (neg[i])
      d[i] = -inf<long long>();
  }
  return d;
}

/* DIJKSTRA */

vector<long long> Dijkstra(int s, int V,
                           vector<tuple<long long, long long, long long>> &G) {
  vector<vector<pair<long long, long long>>> uv(V);
  for (auto &&i : G)
    uv[get<0>(i)].emplace_back(get<1>(i), get<2>(i));
  vector<long long> d(V, inf<long long>() / 2);
  vector<long long> prev(V, -1);
  d[s] = 0;
  priority_queue<pair<long long, long long>, vector<pair<long long, long long>>,
                 greater<pair<long long, long long>>>
      Q;
  rep(i, 0, V) Q.emplace(d[i], i);
  while (!Q.empty()) {
    long long u = Q.top().second;
    Q.pop();
    for (auto &&v : uv[u]) {
      long long alt = d[u] + v.second;
      if (d[v.first] > alt) {
        d[v.first] = alt;
        if (d[v.first] > inf<long long>() / 4)
          d[v.first] = inf<long long>() / 2;
        else if (d[v.first] < -inf<long long>() / 4)
          d[v.first] = -inf<long long>() / 2;
        prev[v.first] = u;
        Q.emplace(alt, v.first);
      }
    }
  }
  for (auto &&i : d) {
    if (abs(i) > inf<long long>() / 4)
      (i *= 2)++;
  }
  return d;
}

/* BFS */

vector<long long> BFS(long long s, long long V,
                      vector<pair<long long, long long>> &G) {
  queue<long long> Q;
  vector<vector<long long>> uv(V);
  for (auto &&i : G)
    uv[i.first].emplace_back(i.second);
  vector<long long> d(V, inf<long long>());
  d[s] = 0;
  Q.emplace(s);
  while (!Q.empty()) {
    long long v = Q.front();
    Q.pop();
    for (auto &&i : uv[v]) {
      if (d[i] == inf<long long>()) {
        d[i] = d[v] + 1;
        Q.emplace(i);
      }
    }
  }
  return d;
}

/* WARSHALLFLOYD */
vector<vector<long long>>
WarshallFloyd(int V, vector<tuple<long long, long long, long long>> &G) {
  vector<vector<long long>> d(V, vector<long long>(V, inf<long long>()));
  for (auto &&i : G) {
    d[get<0>(i)][get<1>(i)] = get<2>(i);
  }
  rep(k, 0, V) rep(i, 0, V) rep(j, 0, V) {
    if (d[i][k] != inf<ll>() && d[k][j] != inf<ll>())
      d[i][j] = min(d[i][j], d[i][k] + d[k][j]);
  }
  rep(i, 0, V) { d[i][i] = 0; }
  return d;
}

/* NUITA.INC & KYOTO UNIVERSITY */

/* LAST EDITED ON 9.5.2019 */

/* CODE STARTS FROM HERE */

int main() {
  fastio(0);
  ll N;
  cin >> N;
  auto P = make_deque<ll>(0);
  rep(i, 0, N) {
    ll p;
    cin >> p;
    P.emplace_back(p);
  }
  auto dir = make_deque<ll>(N);
  rep(i, 0, N) { dir[P[i] - 1] = i; }
  multiset<ll> se;
  rep(i, 0, 2) se.insert(-1);
  rep(i, 0, 2) se.insert(N);
  ll ans{};
  rem(i, N, 0) {
    se.insert(dir[i - 1]);
    auto it = se.lower_bound(dir[i - 1]);
    it--;
    it--;
    ll l2 = *it;
    it++;
    ll l1 = *it;
    it++;
    ll c = *it;
    it++;
    ll r1 = *it;
    it++;
    ll r2 = *it;
    ans += i * ((l1 - l2) * (r1 - c) + (c - l1) * (r2 - r1));
  }
  cout << ans;
}