Language: C++

/* ---------- conversion ---------- */

#define INT(c) static_cast<int>(c)
#define CHAR(n) static_cast<char>(n)
#define LL(n) static_cast<ll>(n)
#define DOUBLE(n) static_cast<double>(n)

/* ---------- container ---------- */

#define ALL(v) (v).begin(), (v).end()
#define SIZE(v) (LL((v).size()))
#define each(itr,v) for(auto (itr):(v))

#define FIND(v, k) (v).find(k) != (v).end()
#define VFIND(v, k) find(ALL(v), k) != (v).end()

#define gsort(b, e) sort(b, e, greater<decltype(*b)>())

/* ----------- debug ---------- */

template <class T>
ostream& operator<<(ostream& os, vector<T> v) {
	os << "[";
	for (auto vv : v)
		os << vv << ",";
	return os << "]";
}

template <class T>
ostream& operator<<(ostream& os, set<T> v) {
	os << "[";
	for (auto vv : v)
		os << vv << ",";
	return os << "]";
}

template <class L, class R>
ostream& operator<<(ostream& os, pair<L, R> p) {
	return os << "(" << p.fst << "," << p.snd << ")";
}

/* ---------- Constants ---------- */

// const ll MOD = 1e9 + 7;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = acos(-1);
// const double EPS = 1e-10;
// mt19937 mert(LL(time(0)));

/* ---------- Short Functions ---------- */

template <typename T>
T sq(T a) {
	return a * a;
}

template <typename T>
T gcd(T a, T b) {
	if (a > b) return gcd(b, a);
	return a == 0 ? b : gcd(b % a, a);
}

template <typename T, typename U>
T mypow(T b, U n) {
	if (n == 0) return 1;
	if (n == 1) return b /* % MOD */;
	if (n % 2 == 0) {
		return mypow(b * b /* % MOD */, n / 2);
	}
	else {
		return mypow(b, n - 1) * b /* % MOD */;
	}
}

ll pcnt(ll b) {
	return __builtin_popcountll(b);
}



// const ll dx[4] = {0, -1, 1, 0};
// const ll dy[4] = {-1, 0, 0, 1};
// const ll dx[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
// const ll dy[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

/* v-v-v-v-v-v-v-v-v Main Part v-v-v-v-v-v-v-v-v */

int main() {

cin.tie(0);
ios::sync_with_stdio(false);

ll N;
cin >> N;
map<long long,int> mp;
vector<long long> edge;
REP(i, N) {
	ll a;
	cin >> a;
	mp[a]++;
}
each(itr, mp) {
	if (itr.second >= 4) edge.push_back(itr.first);
	if (itr.second >= 2) edge.push_back(itr.first);
}
sort(edge.begin(), edge.end());
ll edge1, edge2;
edge1 = edge.back();
edge.pop_back();
edge2 = edge.back();
cout << edge1 * edge2 << endl;
return 0;
}

Compiler output:
a.cc:22:1: error: 'ostream' does not name a type
   22 | ostream& operator<<(ostream& os, vector<T> v) {
      | ^~~~~~~
a.cc:30:1: error: 'ostream' does not name a type
   30 | ostream& operator<<(ostream& os, set<T> v) {
      | ^~~~~~~
a.cc:38:1: error: 'ostream' does not name a type
   38 | ostream& operator<<(ostream& os, pair<L, R> p) {
      | ^~~~~~~
a.cc:76:1: error: 'll' does not name a type; did you mean 'LL'?
   76 | ll pcnt(ll b) {
      | ^~
      | LL
a.cc: In function 'int main()':
a.cc:91:1: error: 'cin' was not declared in this scope
   91 | cin.tie(0);
      | ^~~
a.cc:92:1: error: 'ios' has not been declared
   92 | ios::sync_with_stdio(false);
      | ^~~
a.cc:94:1: error: 'll' was not declared in this scope; did you mean 'LL'?
   94 | ll N;
      | ^~
      | LL
a.cc:95:8: error: 'N' was not declared in this scope
   95 | cin >> N;
      |        ^
a.cc:96:1: error: 'map' was not declared in this scope
   96 | map<long long,int> mp;
      | ^~~
a.cc:96:5: error: expected primary-expression before 'long'
   96 | map<long long,int> mp;
      |     ^~~~
a.cc:97:1: error: 'vector' was not declared in this scope
   97 | vector<long long> edge;
      | ^~~~~~
a.cc:97:8: error: expected primary-expression before 'long'
   97 | vector<long long> edge;
      |        ^~~~
a.cc:98:5: error: 'i' was not declared in this scope
   98 | REP(i, N) {
      |     ^
a.cc:98:1: error: 'REP' was not declared in this scope
   98 | REP(i, N) {
      | ^~~
a.cc:103:11: error: 'mp' was not declared in this scope
  103 | each(itr, mp) {
      |           ^~
a.cc:12:37: note: in definition of macro 'each'
   12 | #define each(itr,v) for(auto (itr):(v))
      |                                     ^
a.cc:104:30: error: 'edge' was not declared in this scope
  104 |         if (itr.second >= 4) edge.push_back(itr.first);
      |                              ^~~~
a.cc:105:30: error: 'edge' was not declared in this scope
  105 |         if (itr.second >= 2) edge.push_back(itr.first);
      |                              ^~~~
a.cc:107:6: error: 'edge' was not declared in this scope
  107 | sort(edge.begin(), edge.end());
      |      ^~~~
a.cc:107:1: error: 'sort' was not declared in this scope; did you mean 'short'?
  107 | sort(edge.begin(), edge.end());
      | ^~~~
      | short
a.cc:108:3: error: expected ';' before 'edge1'
  108 | ll edge1, edge2;
      |   ^~~~~~
      |   ;
a.cc:109:1: error: 'edge1' was not declared in this scope
  109 | edge1 = edge.back();
      | ^~~~~
a.cc:111:1: error: 'edge2' was not declared in this scope
  111 | edge2 = edge.back();
      | ^~~~~
a.cc:112:1: error: 'cout' was not declared in this scope
  112 | cout << edge1 * edge2 << endl;
      | ^~~~
a.cc:112:26: error: 'endl' was not declared in this scope
  112 | cout << edge1 * edge2 << endl;
      |                          ^~~~