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)

var a:array[0..100020] of int64;
var n,ans,x,y,num:int64;
var i:longint;
procedure qsort(l,r:int64);
var i,j,mid,t:int64;
begin
  i:=l;j:=r;mid:=a[(l+r)>>1];
  repeat
    while a[i]>mid do inc(i);
    while a[j]<mid do dec(j);
    if i<=j then
    begin
      t:=a[i];a[i]:=a[j];a[j]:=t;
      inc(i);dec(j);
    end;
  until i>j;
  if l<j then qsort(l,j);
  if i<r then qsort(i,r);
end;
begin
  read(n);
  for i:=1 to n do read(a[i]);qsort(1,n);
  i:=0;
  while i<n do
  begin
    num:=1;inc(i);while a[i]=a[i+1] do begin inc(i);inc(num);end;
    if num>1 then begin if x=0 then x:=a[i] else y:=a[i];end;
    if y>0 then break;
    if (num>=4) then begin y:=a[i];break;end;
  end;
  writeln(x*y);
end.