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 repd(i,a,b) for (int i=(a);i<(b);i++)
#define rep(i,n) repd(i,0,n)
#define all(x) (x).begin(),(x).end()
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
typedef long long ll;
const long long INF = 1LL << 60;
typedef pair<int, int> P;
int main()
{
    int N;
    cin >> N;

    map<int, int> stick;
    vector<int> len;
    rep(i, N) {
        int A;
        cin >> A;
        if (stick.count(A) == 0) {
            stick[A] = 1;
            len.push_back(A);
        }
        else {
            stick[A]++;
        }
    }

    sort(all(len));
    reverse(all(len));

    int cnt = 0;
    ll a1 = 0, a2 = 0;
    rep(i, len.size()) {
        //cout << len[i] << ":" << stick[len[i]] << endl;
        if (stick.count(len[i]) != 0 && stick[len[i]] >= 2) {
            if (stick[len[i]] >= 4) {
                if (a1 == 0) {
                    a1 = len[i];
                    cnt++;
                }
                if (a2 == 0) {
                    a2 = len[i];
                    cnt++;
                }
            }
            if (a1 == 0) {
                a1 = len[i];
                cnt++;
            }
            else if (a2 == 0) {
                a2 = len[i];
                cnt++;
            }
        }
        if (cnt == 2) break;
    }

    if (cnt == 2) {
        cout << 1LL * a1 * a2 << endl;
    }
    else {
        cout << 0 << endl;
    }

    return 0;
}