import java.util.*;
import java.util.Scanner;

class Main {

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        final int N = sc.nextInt();
        long ans = 0;
        for(int i=N;i>0;i--){
            if(Math.floor(Math.log10(i))%2==0){
                ans++;
            }
        }
        System.out.println(ans);
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz