import java.util.*;
import java.io.*;

public class Main
{
	public static void main(String [] Args)
	{
		Scanner in=new Scanner(System.in);
		int n=in.nextInt();
		int ans=0;
		for(int i=1;i<=n;i++)
		{
			int tep=i;
			int len=0;
			while(tep>0)
			{
				tep=tep/10;
				len++;
			}
			if(len%2==1)
			{
				ans++;
			}
		}
		System.out.println(ans);
	}
}
	
/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