program main
  implicit none
  integer::N,i
  integer::V(1000),total(0:1000)
  read *, N
  read *, V(1:N)

  total(0)=0
  total(1)=V(1)

  do i=2,N
     total(i) = MAX(total(i-1),total(i-2)+V(i))
  end do
  print '(i0)',total(N)
end program main