import java.util.Scanner;

public class Main {

  public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);
    int num1 = scanner.nextInt();
    int sum = 0;
    for(int i = 1; i <= num1; i++) {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    }
    System.out.println(sum);
  }
} import java.util.Scanner; Scanner java.util public class Main {

  public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);
    int num1 = scanner.nextInt();
    int sum = 0;
    for(int i = 1; i <= num1; i++) {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    }
    System.out.println(sum);
  }
} Main public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);
    int num1 = scanner.nextInt();
    int sum = 0;
    for(int i = 1; i <= num1; i++) {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    }
    System.out.println(sum);
  } main {

    Scanner scanner = new Scanner(System.in);
    int num1 = scanner.nextInt();
    int sum = 0;
    for(int i = 1; i <= num1; i++) {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    }
    System.out.println(sum);
  } Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int num1 = scanner.nextInt(); num1 scanner.nextInt() scanner.nextInt scanner int sum = 0; sum 0 for(int i = 1; i <= num1; i++) {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    } int i = 1; int i = 1; i 1 i <= num1 i num1 i++ i++ i {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    } {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    } if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	} String.valueOf(i).length() % 2 == 1 String.valueOf(i).length() % 2 String.valueOf(i).length() String.valueOf(i).length String.valueOf(i) String.valueOf String i 2 1 {
    		sum++;
    	} sum++ sum System.out.println(sum) System.out.println System.out System System.out sum String[] args args public class Main {

  public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);
    int num1 = scanner.nextInt();
    int sum = 0;
    for(int i = 1; i <= num1; i++) {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    }
    System.out.println(sum);
  }
} public class Main {

  public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);
    int num1 = scanner.nextInt();
    int sum = 0;
    for(int i = 1; i <= num1; i++) {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    }
    System.out.println(sum);
  }
} Main
import java.util.Scanner;

public class Main {

  public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);
    int num1 = scanner.nextInt();
    int sum = 0;
    for(int i = 1; i <= num1; i++) {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    }
    System.out.println(sum);
  }
}
