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(int i = 0; i < (n); i++)
#define P(x) cout << (x) << "\n"

#define MOD 1000000007 // 1e9+7
#define PI 3.1415926525
#define ll long long int // 10^18
#define INF 1000000001 // 1e9+1

int dx[4]={1,-1,0,0};
int dy[4]={0,0,1,-1};
//---------------------------
ll n,x=0,y=0,a[100005];
bool flag1=true,flag2;

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

  // ifstream in("input.txt");
  // cin.rdbuf(in.rdbuf());

  cin>>n;
  REP(i,n)cin>>a[i];
  sort(a,a+n);
  reverse(a,a+n);
  REP(i,n){
    if(flag1 && a[i]==a[i+1]){
      x=a[i];
      i++;
      flag1=false;
      flag2=true;
    }
    if(flag2&&a[i]==a[i+1]){
      y=a[i];
      break;
    }
  }
  P(x*y);
  return 0;
}