import java.util.*;
public class Main
{
	public static void main(String[] args)
	{
		Scanner in = new Scanner(System.in);
		int n=in.nextInt();
		int c=0;
		for(int i=1;i<=n;i++)
		{
			String str=String.valueOf(i);
			if(str.length()%2!=0)
			{
				c++;
			}
		}
		System.out.println(c);
	}
}

/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