پستimport java.util.Scanner;

import java.util.Scanner; class Solution{ public static void main(String[] args) { Scanner sc=new Scanner(System.in); int n=sc.nextInt(); int count=0; for(int i=1;i<n;i++){ int k=(int) Math.log10(i)+1; if(k%2!=0){ count++; } } System.out.println(count); } }