import java.util.Scanner; 
import java.lang.Math;

public class Main { 
     public static void main(String[] args) { 
         Scanner sc = new Scanner(System.in);
         int n = Integer.parseInt(sc.next());

         System.out.println(solve(n));
     } 
    public static int solve(int n) {
        int ans = 0;
        for (int i = 1; i <= n; i++) {
             int digit = (int) Math.log10(i) + 1;
             if (digit %2==1) {
                ans++;
             } 
         } 
         return 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