import java.util.Scanner;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    sc.close();
    int ans = 0;
    for(int i=1;i<=n;i++) {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		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();
    sc.close();
    int ans = 0;
    for(int i=1;i<=n;i++) {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		ans++;
    	}
    }
    System.out.println(ans);
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    sc.close();
    int ans = 0;
    for(int i=1;i<=n;i++) {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		ans++;
    	}
    }
    System.out.println(ans);
  } main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    sc.close();
    int ans = 0;
    for(int i=1;i<=n;i++) {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		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 sc.close() sc.close sc int ans = 0; ans 0 for(int i=1;i<=n;i++) {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		ans++;
    	}
    } int i=1; int i=1; i 1 i<=n i n i++ i++ i {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		ans++;
    	}
    } {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		ans++;
    	}
    } int dig = (int) (Math.log10(i)+1); dig (int) (Math.log10(i)+1) Math.log10(i) Math.log10 Math i 1 if(dig%2==1) {
    		ans++;
    	} dig%2==1 dig%2 dig 2 1 {
    		ans++;
    	} 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();
    sc.close();
    int ans = 0;
    for(int i=1;i<=n;i++) {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		ans++;
    	}
    }
    System.out.println(ans);
  }
} public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    sc.close();
    int ans = 0;
    for(int i=1;i<=n;i++) {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		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();
    sc.close();
    int ans = 0;
    for(int i=1;i<=n;i++) {
    	int dig = (int) (Math.log10(i)+1);
    	if(dig%2==1) {
    		ans++;
    	}
    }
    System.out.println(ans);
  }
}