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);
    }
} import java.util.*; util.* java 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);
    }
} 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);
    } main {
        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);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc 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); n <= 9 n 9 System.out.println(n) System.out.println System.out System System.out n 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); n <= 99 n 99 System.out.println(9) System.out.println System.out System System.out 9 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); n <= 999 n 999 System.out.println(n-90) System.out.println System.out System System.out n-90 n 90 if(n <= 9999)System.out.println(909);
        else if(n <= 99999)System.out.println(n-9090);
        else System.out.println(90909); n <= 9999 n 9999 System.out.println(909) System.out.println System.out System System.out 909 if(n <= 99999)System.out.println(n-9090);
        else System.out.println(90909); n <= 99999 n 99999 System.out.println(n-9090) System.out.println System.out System System.out n-9090 n 9090 System.out.println(90909) System.out.println System.out System System.out 90909 String[] args args
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);
    }
}