import java.util.*;
class Main{
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count = 0;
    for(int i = 1; i <= N; i++) {
      int test = i;
      int num = 0;
      while( test > 0 ) {
        test /= 10;
        num++;
      }
      if( num % 2 != 0 ){
        count++;
      }
    }
    System.out.println(count);
  }
} import java.util.*; util.* java class Main{
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count = 0;
    for(int i = 1; i <= N; i++) {
      int test = i;
      int num = 0;
      while( test > 0 ) {
        test /= 10;
        num++;
      }
      if( num % 2 != 0 ){
        count++;
      }
    }
    System.out.println(count);
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count = 0;
    for(int i = 1; i <= N; i++) {
      int test = i;
      int num = 0;
      while( test > 0 ) {
        test /= 10;
        num++;
      }
      if( num % 2 != 0 ){
        count++;
      }
    }
    System.out.println(count);
  } main {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count = 0;
    for(int i = 1; i <= N; i++) {
      int test = i;
      int num = 0;
      while( test > 0 ) {
        test /= 10;
        num++;
      }
      if( num % 2 != 0 ){
        count++;
      }
    }
    System.out.println(count);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int count = 0; count 0 for(int i = 1; i <= N; i++) {
      int test = i;
      int num = 0;
      while( test > 0 ) {
        test /= 10;
        num++;
      }
      if( num % 2 != 0 ){
        count++;
      }
    } int i = 1; int i = 1; i 1 i <= N i N i++ i++ i {
      int test = i;
      int num = 0;
      while( test > 0 ) {
        test /= 10;
        num++;
      }
      if( num % 2 != 0 ){
        count++;
      }
    } {
      int test = i;
      int num = 0;
      while( test > 0 ) {
        test /= 10;
        num++;
      }
      if( num % 2 != 0 ){
        count++;
      }
    } int test = i; test i int num = 0; num 0 while( test > 0 ) {
        test /= 10;
        num++;
      } test > 0 test 0 {
        test /= 10;
        num++;
      } test /= 10 test 10 num++ num if( num % 2 != 0 ){
        count++;
      } num % 2 != 0 num % 2 num 2 0 {
        count++;
      } count++ count System.out.println(count) System.out.println System.out System System.out count String[] args args
import java.util.*;
class Main{
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count = 0;
    for(int i = 1; i <= N; i++) {
      int test = i;
      int num = 0;
      while( test > 0 ) {
        test /= 10;
        num++;
      }
      if( num % 2 != 0 ){
        count++;
      }
    }
    System.out.println(count);
  }
}
