let rec f l p n =
        if p + 3 < List.length l then
            let a = (l.[p+0] + l.[p+2])
            let c = (l.[p+0] + l.[p+3])
            let b = (l.[p+1] + l.[p+3])
            let d = List.max [a;b;c]
            let e =
                if d = a || d = c then 0
                else 1
            f l (p+2+e) (n+l.[p+e])
        elif p + 2 < List.length l then
            n + (max (l.[p] + l.[p+2]) l.[p+1])
        elif p + 1 < List.length l then
            n + (max l.[p] l.[p+1])
        elif p < List.length l then
            n + l.[p]
        else n
    stdin.ReadLine() |> ignore
    let l = stdin.ReadLine().Split([|' '|]) |> Seq.map (int) |> Seq.toList
    f l 0 0
    |> (printfn "%d")