import java.util.*;

class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        if(n <= 9) System.out.println(n);
        else if(n <= 99)System.out.println(9);
        else if(n <= 999)System.out.println(n-90);
        else if(n <= 9999)System.out.println(909);
        else if(n <= 99999)System.out.println(n-9090);
        else System.out.println(90909);
    }
}
/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