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

fun member x nil = false
  | member x (h::tl) =
    if x = h
    then true
    else member x tl


val () =
    let
        val n = readInt ()
        val a_s = List.tabulate (n, fn _ => readInt ())

        val ans = List.foldl (fn (a, acc) =>
                                 if member (a - 1) a_s then acc
                                 else a + acc)
                             0
                             a_s
    in
        print (Int.toString ans ^ "\n")
    end