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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<bits/stdc++.h>
#include <queue>
using namespace std;

using ll =  long long;
using pint =  pair<int,int>;

#define rep(i,n) for(int (i)=0,temp=(int)(n);(i)<temp;++(i))
#define repi(i,start,end) for(int (i)=(start),temp=(int)(end);i<(end);++(i))
#define rfor(x,v) for(const auto& (x) : (v))
#define all(x) (x).begin(),(x).end()
#define SORT(v, n) sort((v), (v)+(n))
#define vsort(v) sort((v).begin(), (v).end())
#define vfsort(v,lambda) sort(all((v)),(lambda))
#define vint vector<int>
#define vvint vector<vector<int>> //vvint v(n,vint(n))
#define vin(v) rep(i,(v).size()) {cin >> (v)[i];}
#define int long long
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
//lambda: [&](int x, int y){return x<y;}

signed main(void){
    int n;cin>>n;
    vint a(n);vin(a);
    ll h[2];
    priority_queue<int> q;
    rep(i,n)q.push(a[i]);

    rep(i,2){
        h[i]=q.top();q.pop();
        while(!q.empty()&&h[i]!=q.top()){
            q.pop();
            h[i]=q.top();
        }
        if(q.empty()){
            cout << 0 << endl;
            return 0;
        }
        q.pop();
    } 
    int ans=h[0]*h[1];
    printf("%lld\n",ans);
}