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 Pascal (FPC 2.6.2) (judge verdict: Accepted)

program ec12;
var 
	a,b,site:array[1..100001] of int64;
	j:int64;
	n,m,i,max,tot:longint;
procedure qsrt(l,r:longint);
var 
	s,v:int64;
	i,j:longint;
begin 
	i:=l; j:=r; v:=a[(l+r) div 2];
	repeat 
	while a[i]<v do inc(i);
	while a[j]>v do dec(j);
	if i<=j then 
	begin 
		s:=a[i];
		a[i]:=a[j];
		a[j]:=s;
		inc(i);
		dec(j);
	end;
	until i>=j;
	if l<j then qsrt(l,j);
	if i<r then qsrt(i,r);
end; 
begin 
	readln(n);
	for i:=1 to n do 
	read(a[i]);
	qsrt(1,n);
	m:=0;
	a[n+1]:=0;	
	for i:=n downto 1 do 
	begin
		if a[i]=a[i+1] then 
		begin 
			if m=0 then 
			begin 
				m:=1;
				j:=a[i];
				a[i]:=-1;
			end
			else
			begin 
				writeln(j*a[i]);
				halt;
			end;
		end;
	end;
	writeln(0);
end.