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(obj) (obj).begin(), (obj).end()
#define FOR(i, a, b) for(int i = (a); i < (b); ++i)
#define REP(i, n) for(int i = 0; i < (n); ++i)

#define endl "\n"
#define pb push_back
#define elif else if
#define cout_db cout << fixed

#define MOD 1000000007
 
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1};
const int dx_8[]={1,1,0,-1,-1,-1,0,1}, dy_8[]={0,-1,-1,-1,0,1,1,1};

typedef long long ll;

template <typename T>
inline T GCD(T a,T b){T c;while(b!=0){c=a%b;a=b;b=c;}return a;}
template <typename T>
inline T LCM(T a,T b){T c=GCD(a,b);a/=c;return a*b;}
template <typename T>
inline T nCr(T a,T b){T i,r=1;for(i=1;i<=b;i++){r*=(a+1-i);r/=i;}return r;}
template <typename T>
inline T nHr(T a,T b){return nCr(a+b-1,b);}
template <typename T>
inline T POW(T a,T b){int i,r=1;for(i=1;i<=b;i++){r*=a;}return r;}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	int n;
	ll a[100000];
	cin >> n;
	REP(i, n) cin >> a[i];

	map<ll, ll> mp;
	REP(i, n) mp[a[i]]++;

	sort(a, a+n);

	for(int i = n-1;i >= 0;i--){
		if(mp[a[i]] >= 4){
			cout << a[i] * a[i] << endl;
			return 0;
		}
		else if(mp[a[i]] >= 2){
			for(int j = n-1;j >= 0;j--){
				if(a[j] < a[i] and mp[a[j]] >= 2){
					cout << a[i] * a[j] << endl;
					return 0;
				}
			}
		}
	}
	cout << 0 << endl;
	
	return 0;
}