import java.util.*;
public class Main {
public static void main(String args[])
{
  Scanner sp=new Scanner(System.in);
    int n=sp.nextInt();
    int sc=0;
    int l=(int) (Math.log10(n) + 1);
 
    if(l%2==0)
    
     {
     for(int i=n;i>0;i--)
     {
        int sn=(int) (Math.log10(i) + 1);
        if(!(sn%2==0))
         {
          sc++;  
         }
     }
      System.out.println(sc);
    }
    else
    {
     for(int i=n;i>0;i--)
     {
        int sn=(int) (Math.log10(i) + 1);
        if(!(sn%2==0))
         {
          sc++;  
         }
     }
      System.out.println(sc);
    }
}
}
	
	
	

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz