import java.util.Scanner;

public class Main {

	
	public static void main(String[] args) {
		
		
		Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        	
				
		if(n<=9) {
			System.out.println(n);
		}else if(n<=99){
			System.out.println(9);
		}else if(n<=999) {
			System.out.println(n-90);
	    }else if(n<=9999) {
	    	System.out.println(909);
	    }else if(n<=99999) {
	    	System.out.println(n-9090);
	    }else{
	    	System.out.println(90909);
	    }
		
	}

}

/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