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;

long long n,a[100005];
map<long long, int> cnt;
vector<long long> v;

main(){
			ios::sync_with_stdio(false); cin.tie(0);
			//freopen("input.inp","r",stdin);
			cin>>n;
			for(int i=1; i<=n; i++){
				cin>>a[i];
				if (cnt[a[i]]==0) v.push_back(a[i]);
				cnt[a[i]]++;
			}
			
			long long max1=0, max2=0;
			for(int i=0; i<v.size(); i++){
				if (cnt[v[i]]>=2){
					if (v[i]>=max1){
						max2 = max1;
						max1 = v[i];
					}
					else{
						if (v[i]>=max2) max2 = v[i];
					}
				}
			}
			long long res;
			if (max2==0 && max1!=0) res = max1*max1;
			else res=max1*max2;
			cout<<res;
}