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 ans = 0;
        for (int i = 1; i < n + 1; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        }
        System.out.println(ans);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        sc.close();
        int ans = 0;
        for (int i = 1; i < n + 1; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        }
        System.out.println(ans);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        sc.close();
        int ans = 0;
        for (int i = 1; i < n + 1; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        }
        System.out.println(ans);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        sc.close();
        int ans = 0;
        for (int i = 1; i < n + 1; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        }
        System.out.println(ans);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc sc.close() sc.close sc int ans = 0; ans 0 for (int i = 1; i < n + 1; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        } int i = 1; int i = 1; i 1 i < n + 1 i n + 1 n 1 i++ i++ i {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        } {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        } if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            } String.valueOf(i).length() % 2 == 1 String.valueOf(i).length() % 2 String.valueOf(i).length() String.valueOf(i).length String.valueOf(i) String.valueOf String i 2 1 {
                ans++;
            } ans++ ans System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        sc.close();
        int ans = 0;
        for (int i = 1; i < n + 1; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        }
        System.out.println(ans);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        sc.close();
        int ans = 0;
        for (int i = 1; i < n + 1; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        }
        System.out.println(ans);
    }
} Main
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 ans = 0;
        for (int i = 1; i < n + 1; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                ans++;
            }
        }
        System.out.println(ans);
    }
}