program main
   implicit none
   integer(8) n,beki(31)
   integer(8),allocatable,dimension(:)::a
   integer(8) ::i,j,k,counter=0
   read(*,*)n
   allocate(a(n))
   read(*,*)a
  ! call heapsort(n,a)
   do i=1,31
      beki(i)=2**i
   enddo
 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
   do i=n,2,-1
      do j=i-1,1,-1
         do k=1,31
            if(beki(k)==a(i)+a(j) .and. a(i)/=0 .and. a(j)/=0)then
               counter=counter+1
               a(j)=0
            endif
         enddo
      enddo
   enddo
   write(*,*)counter
contains
subroutine heapsort(n,array)
  implicit none
  integer(8),intent(in) :: n
  integer(8),intent(inout) :: array(1:n)
  
  integer(8) ::i,k,j,l
  integer(8) :: 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 main
