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 INF 1e9
#define rep(i, n) for(int i = 0; i < n; i++)
#define ALL(x) (x).begin(), (x).end()  //昇順
#define RALL(x) (x).rbegin(), (x).rend() // 降順
const long long mod = 1e9+7;
typedef priority_queue<int> PQ;
typedef vector<long long> VL;
typedef vector<int> VI;  // VI a(n);
typedef vector<string> VS;  
typedef vector<char> VC; 
typedef vector<VS> VSS;
typedef vector<VC> VCC;    
typedef vector<VI> VII;   // VII a(n,vector<int>(m)) n * m 
typedef vector<VL> VLL;
typedef pair<int, int> PII;
typedef map<int, int> MP;   //MP a;
typedef vector<pair<int, int> > PS;
typedef long long ll;  //ll とdoubleは違う
template <class T>  // chmax(max, a);
bool chmax(T &a, const T &b)
{
    if (a < b)
    {
        a = b;
        return 1;
    }
    return 0;
}
template <class T> //chmin(min,a)
bool chmin(T &a, const T &b)
{
    if (b < a)
    {
        a = b;
        return 1;
    }
    return 0;
}
template<typename T>   // operator << (cout,a);
ostream& operator << (ostream& os, vector<T>& v) {
    os << "{";
    rep(i, (int)v.size()) { os << v[i] << (i < v.size() - 1 ? ", " : ""); }
    os << "}";
    return os;
}
template<typename T>
bool all_same(vector<T> v1,vector<T> v2){
    sort(ALL(v1));
    sort(ALL(v2),greater<T>());
    return v1 == v2;
}
 
// g++ -std=c++11 prac.cpp
ll Max=-1,Min=1e9+1;
 
int main(){
    ll x=0,y=0,n,m,k,l,r,w,sum1=0,sum2=0,ans=0;
    string s,t;
    bool f=true;
    map <int,int> a;
    cin >> n;
    rep(i,n){
        cin >> x;
        if (a.find(x) != a.end()){
            a.find(x)->second++;
        }
        else{
            a.insert(make_pair(x,1));
        }
    }
    for( auto i = a.begin(); i != a.end() ; ++i ) {      //for文で回す。
        if(i->second >= 2){
            if(x < i->first){
                y = x;
                x = i->first;
            }
            if (y < i->first && x > i->first){
                y = i->first;
            }
        } 
    }

    cout << x*y << endl;
    return 0;
}