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=N;i>0;i--){
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    }
 
  	System.out.println(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=N;i>0;i--){
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    }
 
  	System.out.println(cnt);
  
	}
} Main public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
  	int N = sc.nextInt();
 	int cnt = 0;	

  	for (int i=N;i>0;i--){
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    }
 
  	System.out.println(cnt);
  
	} main {
	Scanner sc = new Scanner(System.in);
  	int N = sc.nextInt();
 	int cnt = 0;	

  	for (int i=N;i>0;i--){
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         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=N;i>0;i--){
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    } int i=N; int i=N; i N i>0 i 0 i-- i-- i {
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    } {
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    } if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      } ((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000)) ((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000)) ((0<i)&&(i<10)) (0<i) 0 i (i<10) i 10 ((99<i)&&(i<1000)) (99<i) 99 i (i<1000) i 1000 ((9999<i)&&(i<100000)) (9999<i) 9999 i (i<100000) i 100000 {
         cnt++;
      } cnt++ cnt System.out.println(cnt) System.out.println System.out System System.out cnt String[] args args 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=N;i>0;i--){
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    }
 
  	System.out.println(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=N;i>0;i--){
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    }
 
  	System.out.println(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=N;i>0;i--){
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    }
 
  	System.out.println(cnt);
  
	}
}