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 FOR(i,l,r) for(long long i=(l);i<(r);++i)
#define REP(i,n) FOR(i,0,n)
#define int long long
#define _GLIBCXX_DEBUG
const int INF=1e15;

signed main(){
    int N;cin>>N;
    map<int,int>memo;
    REP(i,N){
        int a;cin>>a;
        memo[a*(-1)]++;
    }
    int ans=0;
    int count=0;
    for(auto it=memo.begin();it!=memo.end()&&count<=1;it++){
      	if((it->second)<2)continue;
        if(ans==0)ans=it->first;
        else ans*=it->first;
        count++;
    }
    cout<<ans<<endl;
}