import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int digits = String.valueOf(n).length();
        int ans = 0;
        for (int i = 1; i <= digits; i++) {
            if (i % 2 == 1) {
                if (i == digits) {
                    ans += n - (int)Math.pow(10, i-1) + 1;
                } else {
                    ans += (int)Math.pow(10, i-1) * 9;
                }
            }
        }
        // 出力
        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