import java.util.*;
public class Main {

  public static void main(String[] args) { 
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      int cnt = 0;
      for(int i = 1; i <= n; i++) {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      }
      System.out.println(cnt);
  }

  private static int numDigit(int n) {
    int cnt = 0;

    while(n > 0) {
      cnt++;
      n /= 10;
    }
    return cnt;
  }
} import java.util.*; util.* java public class Main {

  public static void main(String[] args) { 
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      int cnt = 0;
      for(int i = 1; i <= n; i++) {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      }
      System.out.println(cnt);
  }

  private static int numDigit(int n) {
    int cnt = 0;

    while(n > 0) {
      cnt++;
      n /= 10;
    }
    return cnt;
  }
} Main public static void main(String[] args) { 
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      int cnt = 0;
      for(int i = 1; i <= n; i++) {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      }
      System.out.println(cnt);
  } main { 
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      int cnt = 0;
      for(int i = 1; i <= n; i++) {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      }
      System.out.println(cnt);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int cnt = 0; cnt 0 for(int i = 1; i <= n; i++) {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      } int i = 1; int i = 1; i 1 i <= n i n i++ i++ i {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      } {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      } int digit = numDigit(i); digit numDigit(i) numDigit i if(digit % 2 == 1)
          cnt++; digit % 2 == 1 digit % 2 digit 2 1 cnt++ cnt System.out.println(cnt) System.out.println System.out System System.out cnt String[] args args private static int numDigit(int n) {
    int cnt = 0;

    while(n > 0) {
      cnt++;
      n /= 10;
    }
    return cnt;
  } numDigit {
    int cnt = 0;

    while(n > 0) {
      cnt++;
      n /= 10;
    }
    return cnt;
  } int cnt = 0; cnt 0 while(n > 0) {
      cnt++;
      n /= 10;
    } n > 0 n 0 {
      cnt++;
      n /= 10;
    } cnt++ cnt n /= 10 n 10 return cnt; cnt int n n public class Main {

  public static void main(String[] args) { 
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      int cnt = 0;
      for(int i = 1; i <= n; i++) {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      }
      System.out.println(cnt);
  }

  private static int numDigit(int n) {
    int cnt = 0;

    while(n > 0) {
      cnt++;
      n /= 10;
    }
    return cnt;
  }
} public class Main {

  public static void main(String[] args) { 
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      int cnt = 0;
      for(int i = 1; i <= n; i++) {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      }
      System.out.println(cnt);
  }

  private static int numDigit(int n) {
    int cnt = 0;

    while(n > 0) {
      cnt++;
      n /= 10;
    }
    return cnt;
  }
} Main
import java.util.*;
public class Main {

  public static void main(String[] args) { 
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      int cnt = 0;
      for(int i = 1; i <= n; i++) {
        int digit = numDigit(i);
        if(digit % 2 == 1)
          cnt++;
      }
      System.out.println(cnt);
  }

  private static int numDigit(int n) {
    int cnt = 0;

    while(n > 0) {
      cnt++;
      n /= 10;
    }
    return cnt;
  }
}