import java.util.Scanner;

public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int ans = 0;
    for(int i = 1;i <= n;i++){
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    }
    System.out.println(ans);
  }
} import java.util.Scanner; Scanner java.util public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int ans = 0;
    for(int i = 1;i <= n;i++){
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    }
    System.out.println(ans);
  }
} Main public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int ans = 0;
    for(int i = 1;i <= n;i++){
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    }
    System.out.println(ans);
  } main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int ans = 0;
    for(int i = 1;i <= n;i++){
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    }
    System.out.println(ans);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int ans = 0; ans 0 for(int i = 1;i <= n;i++){
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    } int i = 1; int i = 1; i 1 i <= n i n i++ i++ i {
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    } {
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    } if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++; i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000 i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000 i < 10 i 10 i >= 100 && i < 1000 i >= 100 i 100 i < 1000 i 1000 i >= 10000 && i < 100000 i >= 10000 i 10000 i < 100000 i 100000 ans++ ans System.out.println(ans) System.out.println System.out System System.out ans String args[] args public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int ans = 0;
    for(int i = 1;i <= n;i++){
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    }
    System.out.println(ans);
  }
} public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int ans = 0;
    for(int i = 1;i <= n;i++){
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    }
    System.out.println(ans);
  }
} Main
import java.util.Scanner;

public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int ans = 0;
    for(int i = 1;i <= n;i++){
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    }
    System.out.println(ans);
  }
}
