import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    int N = in.nextInt();

    int ans = 0;
    if (N < 10) {
      ans = N;
    } else if (N < 100) {
      ans = 9;
    } else if (N < 1000) {
      ans = 9;
      ans += N - 99;
    } else if (N < 10000) {
      ans = 909;
    } else if (N < 100000) {
      ans = 909;
      ans += N - 9999;
    } else if (N < 1000000) {
      ans = 90909;
    }

    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