import java.util.*;
public class Main {
		
	public static void main(String[] args) {		
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.close();
		
		int count = 0;
		for(int i=1;i<=N;i++) {
			
			//111 -> 11 -> 1 -> 0
			int cnt = 0;
			
			int tmp = i;
			
			while(tmp != 0) {
				tmp/=10; // i = i / 10 ;  i = i + 3 -> i+=3; i = i+1 -> i++
				cnt ++;
			}
			
			if(cnt % 2 == 1) count ++;
			
		}
		System.out.println(count);
	}
	
}
/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