fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)


val () =
    let
        val n = readInt ()
        val vs = Array.tabulate (n, fn _ => readInt ())
        val dp = Array.array (n + 1, 0)

        fun doDp index =
            if index <= n
            then
                (
                  Array.update (dp, index, Int.max (Array.sub (dp, index - 1), Array.sub (dp, index - 2) + Array.sub (vs, index - 1)));
                  doDp (index + 1)
                )
            else ignore () 
    in
        if n = 1 then print (Int.toString (Array.sub (vs, 0)) ^ "\n")
        else
            (
              Array.update (dp, 1, Array.sub (vs, 0));
              doDp 2;
              print (Int.toString (Array.sub (dp, n)) ^ "\n")
            )
    end