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>
using namespace std;

#define rep(i,n) for(long long i=0; i<(long long)(n);i++)
#define rep2(i,s,n) for(long long i=(s); i<(long long)(n);i++)

#define mod 1e9+7;
#define inf 1e18+7;

typedef long long ll;

int main() {
    int n;cin>>n;

    vector<ll>a(n+1);
    a[n]=mod;
    rep(i,n){
        cin>>a[i];
    }

    sort(a.begin(),a.end());

    vector<pair<ll,ll>>A(n);
    int cnt=1;

    rep(i,n){
        if(a[i]==a[i+1]){
            cnt++;
        }
        else {
            A[i].first=a[i];
            A[i].second=cnt;
            cnt=1;
        }
    }

    sort(A.begin(),A.end());
    reverse(A.begin(),A.end());

    ll ans=1,cnt2=0;

    rep(i,n){
        if(A[i].second>=4){
            ans=A[i].first*A[i].first;
            cnt2=2;
            break;
        }
        if(A[i].second>=2){
            ans*=A[i].first;
            cnt2++;
        }
        if(cnt2==2){
            break;
        }
    }

    if(cnt2<2){
        cout<<0<<endl;
        return 0;
    }

    cout<<ans<<endl;
}