Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in C++ (GCC 9.2.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define all(v) v.begin(),v.end()
#define len(x) (ll)(x).length()
#define maxs(x,y) x = max(x,y)
#define mins(x,y) x = min(x,y)
#define pb push_back
#define sz(x) (ll)(x).size()
#define v(T) vector<T>
#define vv(T) vector<vector<T>>
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
ll gcd(ll a,ll b){if(a%b==0){return b;}else{return(gcd(b,a%b));}}
ll lcm(ll a,ll b){return a*b/gcd(a,b);}
const int INF=1e9;
const ll INFS=1e18;
const int MOD=INF+7;
const int di[] = {-1,0,1,0};
const int dj[] = {0,-1,0,1};
const double PI=acos(-1);
 
int main() {
  ll n;
  cin>>n;
  vi cnt;
  ll ans=0;
  map<ll,ll> mp;
  rep(i,n){
    ll a;
    cin>>a;
    mp[a]++;
    if(mp[a]==2){
      cnt.pb(a);
      mp[a]=0;
    }
  }
  if(sz(cnt)==0){
    cout<<0<<endl;
    return 0;
  }
  if(sz(cnt)==1){
    cout<<0<<endl;
    return 0;
  }
  sort(all(cnt));
  reverse(all(cnt));
  ans=cnt[0]*cnt[1];
  cout<<ans<<endl;
}