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())

seq {
    for x in [ 1 .. N ] do
        if string(x).Length % 2 = 1 then yield 1
}
|> Seq.sum
|> print
