import java.util.*;

class Main {
    //
    static final long MOD = 1_000_000_007; // 10^9+7
    static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9

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

        int ans = 0;
        for(int i = 1;i <= n;i++){
            if(i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000)){
                ans++;
            }
        }
        System.out.println(ans);
    }

} import java.util.*; util.* java class Main {
    //
    static final long MOD = 1_000_000_007; // 10^9+7
    static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9

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

        int ans = 0;
        for(int i = 1;i <= n;i++){
            if(i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000)){
                ans++;
            }
        }
        System.out.println(ans);
    }

} Main //
    static final long MOD = 1_000_000_007; // 10^9+7 MOD 1_000_000_007 static final int MAX = 2_147_483_646; // intの最大値 MAX 2_147_483_646 static final int INF = 1_000_000_000; // 10^9 INF 1_000_000_000 public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int ans = 0;
        for(int i = 1;i <= n;i++){
            if(i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000)){
                ans++;
            }
        }
        System.out.println(ans);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int ans = 0;
        for(int i = 1;i <= n;i++){
            if(i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000)){
                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 int ans = 0; ans 0 for(int i = 1;i <= n;i++){
            if(i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000)){
                ans++;
            }
        } int i = 1; int i = 1; i 1 i <= n i n i++ i++ i {
            if(i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000)){
                ans++;
            }
        } {
            if(i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000)){
                ans++;
            }
        } if(i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000)){
                ans++;
            } i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000) i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000) i < 10 i 10 (i >= 100&&i < 1000) i >= 100 i 100 i < 1000 i 1000 (i >= 10000&&i < 100000) i >= 10000 i 10000 i < 100000 i 100000 {
                ans++;
            } ans++ ans System.out.println(ans) System.out.println System.out System System.out ans String args[] args
import java.util.*;

class Main {
    //
    static final long MOD = 1_000_000_007; // 10^9+7
    static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9

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

        int ans = 0;
        for(int i = 1;i <= n;i++){
            if(i < 10||(i >= 100&&i < 1000)||(i >= 10000&&i < 100000)){
                ans++;
            }
        }
        System.out.println(ans);
    }

}