#include <bits/stdc++.h>
#define rep(i,x,n) for(int i=x; i<(int)(n); i++)
#define rep_eq(i,x,n) for(int i=x; i<=(int)(n); i++)
using namespace std;

using ll=int64_t;

int main() {
  int N; cin >>N;
  vector<ll> check;
  rep(i,0,N) {
    ll A; cin >>A;
    check.push_back(A);
  }
  sort(check.rbegin(),check.rend());
  ll temp=0;
  rep(i,1,N) {
    if (check[i-1]==check[i]) {
      if (!temp) {
        temp=check[i];
        i++;
      } else {
        cout <<check[i]*temp <<endl;
        return 0;
      }
    }
  }
  cout <<0 <<endl;
  return 0;
}

#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define int long long
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define rev(i, n) for(int i = (int)(n - 1); i >= 0; i--)
#define rev1(i, n) for(int i = (int)(n); i > 0; i--)
#define pb push_back
#define all(v) (v).begin(), (v).end()
#define resort(v) sort((v).rbegin(), (v).rend())
#define vi vector<int>
#define vvi vector<vector<int>>
#define vc vector<char>
#define vvc vector<vector<char>>
#define vb vector<bool>
#define vvb vector<vector<bool>>
using ll = long long;
using P = pair<int, int>;
/* ----------------よく使う数字や配列----------------- */
int dx[] = {1,0,-1,0};
int dy[] = {0,1,0,-1};
constexpr ll mod = 1e9+7;
constexpr int inf = INT32_MAX/2;
constexpr ll INF = LLONG_MAX/2;
constexpr long double eps = DBL_EPSILON;
constexpr long double pi = 3.141592653589793238462643383279;
/* ----------------------end----------------------- */
 
/* --------------------テンプレート------------------ */
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
/* ----------------------end----------------------- */
 
/* --------------------ライブラリ-------------------- */
ll fact(int i) {       //階乗
    if (i == 0) return 1;
    return (fact(i - 1)) * i % mod;
}
ll gcd(ll a, ll b) {        //最大公約数
    if(b == 0) return a;
    return gcd(b, a % b); 
}
ll lcm(ll a, ll b) {      //最小公倍数
    return a / gcd(a, b) * b ;
}
int keta(ll n) {        //桁数を求める
    if(n == 0) return 1;
    int count = 0;
    while(n != 0) {
        n /= 10;
        count++;
    }
    return count;
}
ll ketasum(ll n) {    //各桁の和
    ll sum = 0;
    while(n != 0) {
        sum += n % 10;
        n /= 10;
    }
    return sum;
}
/* ----------------------end----------------------- */

signed main() {
  int n;cin >> n;
  map<int,int> mp;
  rep(i,n) {
	  int a;cin >> a;
	  mp[a]++;
  }
  vector<P> v;
  for(auto p:mp) v.pb({p.first,p.second});
  resort(v);
  //for(auto p:v) cout << p.first << " " << p.second << endl;
  int k=0,l=0;
  rep(i, v.size()) {
	  //cout << "OK" << endl;
	  if(k>0&&l>0) break;
	  if(k==0&&l==0) {
		  if(v[i].second>=4) k=v[i].first,l=v[i].first;
		  else if(v[i].second>=2) k=v[i].first;
	  }
	  else if(k>0) {
		  if(v[i].second>=2) l=v[i].first;
	  }
  }
  cout << k*l << endl;
  return 0;
}