program select_half
  implicit none
  integer(8), parameter :: inf = 1e17
  integer :: n, i, j
  integer(8) :: x = -inf
  integer(8), dimension(-1:200002) :: a = 0_8, l = 0_8, r = 0_8
  integer(8), dimension(-1:200002) :: lm = inf, rm = inf
  logical :: is_even = .false.
  read(*,*) n
  read(*,*) a(1:n)
  do i = 1, n
    l(i) = l(i-2)+a(i)
    lm(i) = min(lm(i-2),a(i))
  end do
  do i = n, 1, -1
    r(i) = r(i+2)+a(i)
    rm(i) = min(rm(i+2),a(i))
  end do
  lm(-1:0) = 0_8
  lm(n+1:n+2) = 0_8
  rm(-1:0) = 0_8
  rm(n+1:n+2) = 0_8
  is_even = mod(n,2) == 0
  do i = 0, n
    x = max(x,l(i-1)+r(i+2))
    x = max(x,l(i)+r(i+2))
    x = max(x,l(i-1)+r(i+1))
    if (is_even) cycle
    x = max(x,l(i)+r(i+1)-a(i))
    x = max(x,l(i)+r(i+1)-a(i+1))
    if (mod(i,2) == 1) cycle
    do j = i+4, n, 2
      x = max(x,l(i-1)+r(i+2)-r(j)+r(j+1))
    end do
  end do
  write(*,'(i0)') x
end program select_half