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: Accepted)

#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<n;i++)
#define INF 1000000000
#define mod 1000000007
typedef long long ll;
const ll LINF = 1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
int gcd(int a,int b){return b?gcd(b,a%b):a;}

int main(){
    int n;
    cin>>n;
    map<ll,ll,greater<ll>> m;
    rep(i,n){
        ll a;
        cin>>a;
        m[a]++;
    }
    ll e1=0,e2=0;
    for(auto x: m){
        if(x.second>=4){
            if(e1==0&&e2==0){
                e1=x.first;e2=x.first;
            }
            else{
                e2=x.first;
            }
        }
        else if(x.second>=2){
            if(e1==0){
                e1=x.first;
            }
            else{
                e2=x.first;
            }
        }
        if(e1>0&&e2>0) break;
    }
    cout<<e1*e2<<endl;
    return 0;
}