PROGRAM MAIN
    IMPLICIT NONE
    INTEGER :: N, I
    INTEGER, ALLOCATABLE, DIMENSION(:) :: V

    READ *, N
    
    ALLOCATE(V(N))
    
    READ *, V(:)

    IF (N <= 2) THEN
        PRINT *, MAXVAL(V)
        RETURN
    END IF

    V(3) = V(1) + V(3)

    DO I = 4, N
        V(I) = V(I) + MAXVAL((/V(I-2), V(I-3)/))
    END DO

    PRINT *, MAXVAL((/V(N), V(N-1)/))
END PROGRAM MAIN