import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    if(N<10){System.out.println(N);}
    else if(N<100){System.out.println(9);}
    else if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);}
  }
} import java.util.*; util.* java public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    if(N<10){System.out.println(N);}
    else if(N<100){System.out.println(9);}
    else if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){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<10){System.out.println(N);}
    else if(N<100){System.out.println(9);}
    else if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);}
  } main {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    if(N<10){System.out.println(N);}
    else if(N<100){System.out.println(9);}
    else if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){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<10){System.out.println(N);}
    else if(N<100){System.out.println(9);}
    else if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);} N<10 N 10 {System.out.println(N);} System.out.println(N) System.out.println System.out System System.out N if(N<100){System.out.println(9);}
    else if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);} N<100 N 100 {System.out.println(9);} System.out.println(9) System.out.println System.out System System.out 9 if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);} N<1000 N 1000 {System.out.println(N-90);} System.out.println(N-90) System.out.println System.out System System.out N-90 N 90 if(N<10000){System.out.println(909);}
    else if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);} N<10000 N 10000 {System.out.println(909);} System.out.println(909) System.out.println System.out System System.out 909 if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);} N<100000 N 100000 {System.out.println(N-9090);} System.out.println(N-9090) System.out.println System.out System System.out N-9090 N 9090 {System.out.println(90909);} System.out.println(90909) System.out.println System.out System System.out 90909 String[] args args public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    if(N<10){System.out.println(N);}
    else if(N<100){System.out.println(9);}
    else if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);}
  }
} public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    if(N<10){System.out.println(N);}
    else if(N<100){System.out.println(9);}
    else if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);}
  }
} Main
import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    if(N<10){System.out.println(N);}
    else if(N<100){System.out.println(9);}
    else if(N<1000){System.out.println(N-90);}
    else if(N<10000){System.out.println(909);}
    else if(N<100000){System.out.println(N-9090);}
    else{System.out.println(90909);}
  }
}