program prob5
    implicit none
    integer(16)::N, i, j, cnt, ans1
    integer(16),allocatable::A(:), c(:), d(:)
    read(*,*) N
    allocate(A(N), c(N), d(N))
    read(*,*) A
    call heapsort(N,A)

    j = 1
    cnt = 0
    c(1) = 1
    d(1) = A(N)
    do i = N-1, 1, -1
        if(A(i) == d(j))then
            c(j) = c(j) + 1
            if(cnt == 1 .and. c(j) >= 4)then
                write(*,*) A(i)*A(i)
                stop
            else if(c(j) == 2)then
                if(cnt == 0)then
                    cnt = 1
                    ans1 = d(j)
                else if(cnt == 1)then
                    write(*,*) ans1*d(j)
                    stop
                end if
            end if
        else
            j = j+1
            c(j) = 1
            d(j) = A(i)
        end if

    end do
    write(*,*) 0
    stop
    contains
    subroutine heapsort(n,array)
        implicit none
        integer(16),intent(in) :: n
        integer(16),intent(inout) :: array(1:n)
       
        integer ::i,k,j,l
        double precision :: t
       
        if(n.le.0)then
           write(6,*)"Error, at heapsort"; stop
        endif
        if(n.eq.1)return
      
        l=n/2+1
        k=n
        do while(k.ne.1)
           if(l.gt.1)then
              l=l-1
              t=array(L)
           else
              t=array(k)
              array(k)=array(1)
              k=k-1
              if(k.eq.1) then
                 array(1)=t
                 exit
              endif
           endif
           i=l
           j=l+l
           do while(j.le.k)
              if(j.lt.k)then
                 if(array(j).lt.array(j+1))j=j+1
              endif
              if (t.lt.array(j))then
                 array(i)=array(j)
                 i=j
                 j=j+j
              else
                 j=k+1
              endif
           enddo
           array(i)=t
        enddo
      
        return
    end subroutine heapsort
end program