open System.Linq
open System.Text.RegularExpressions

let input() = stdin.ReadLine()
let inputs() = input().Split()

let YN b = printfn "%s" (if b then "Yes" else "No")
let print x = printfn "%A" x
let printS (s: string) = printfn "%s" s


let N = int (input())

[ 1 .. N ]
|> Seq.sumBy (fun x -> string(x).Length % 2)
|> print
