import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);
        int n = Integer.parseInt(in.nextLine());

        int count = 0;
        if( n < 10 ) {
            System.out.println(n);
            return;
        }
        if( n < 100 ){
            System.out.println( 9 );
            return;
        }
        if( n < 1000 ){
            System.out.println( n - 99 + 9 );
            return;
        }
        if( n < 10000 ){
            System.out.println( 999 - 99 + 9);
            return;
        }
        if( n < 100000 ){
            System.out.println( n - 9999 + 999 - 99 + 9);
            return;
        }
        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