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;
using ll = long long;
using ld = long double;

#define DUMP(x) cout << #x << " = " << (x) << endl;
#define REP(i, n) for(size_t i = 0; i < (n); i++)
#define REPR(i, n) for(size_t i = (n); i >= 0; i--)
#define FOR(i, a, b) for(size_t i = (a); i < (b); i++)
#define FOREACH(x, a) for(auto& (x) : (a))

const int INF = 1e9;
const int MOD = 1e9 + 7;

int N;
vector<int> A;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);

    cin >> N;
    REP(i, N) {
        int Ai; cin >> Ai;
        A.push_back(Ai);
    }

    sort(A.begin(), A.end(), greater<int>());
    int s1 = 0; // 長い辺
    int s2 = 0; // 短い辺
    REP(i, N) {
        if(A[i] == A[i + 1]) {
            swap(s1, s2);
            s2 = A[i];
            i++;
        }
        if (s1 > 0) break;
    }

    cout << (ll)s1 * s2 << "\n";
}