program main
  implicit none
  integer::N,i,temp
  integer::V(1001),total(0:1001)
  read *, N
  read *, V(1:N)

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

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