import java.util.Scanner;

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

        int n = sc.nextInt();

        int[] num = {0, 10, 100, 1000, 10000, 100000};
        int[] cnt = {0, 9, 10, 909, 910, 90909};

        int i = digitCnt(n);
        if (i % 2 == 0) {
            System.out.println(cnt[i-1]);
        } else {
            System.out.println(cnt[i-1] + n - num[i-1]);
        }
    }

    static int digitCnt(int n) {
        int cnt = 0;
        while (n >= 10) {
            n /= 10;
            cnt++;
        }

        return cnt+1;
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        int[] num = {0, 10, 100, 1000, 10000, 100000};
        int[] cnt = {0, 9, 10, 909, 910, 90909};

        int i = digitCnt(n);
        if (i % 2 == 0) {
            System.out.println(cnt[i-1]);
        } else {
            System.out.println(cnt[i-1] + n - num[i-1]);
        }
    }

    static int digitCnt(int n) {
        int cnt = 0;
        while (n >= 10) {
            n /= 10;
            cnt++;
        }

        return cnt+1;
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        int[] num = {0, 10, 100, 1000, 10000, 100000};
        int[] cnt = {0, 9, 10, 909, 910, 90909};

        int i = digitCnt(n);
        if (i % 2 == 0) {
            System.out.println(cnt[i-1]);
        } else {
            System.out.println(cnt[i-1] + n - num[i-1]);
        }
    } main {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        int[] num = {0, 10, 100, 1000, 10000, 100000};
        int[] cnt = {0, 9, 10, 909, 910, 90909};

        int i = digitCnt(n);
        if (i % 2 == 0) {
            System.out.println(cnt[i-1]);
        } else {
            System.out.println(cnt[i-1] + n - num[i-1]);
        }
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int[] num = {0, 10, 100, 1000, 10000, 100000}; num {0, 10, 100, 1000, 10000, 100000} 0 10 100 1000 10000 100000 int[] cnt = {0, 9, 10, 909, 910, 90909}; cnt {0, 9, 10, 909, 910, 90909} 0 9 10 909 910 90909 int i = digitCnt(n); i digitCnt(n) digitCnt n if (i % 2 == 0) {
            System.out.println(cnt[i-1]);
        } else {
            System.out.println(cnt[i-1] + n - num[i-1]);
        } i % 2 == 0 i % 2 i 2 0 {
            System.out.println(cnt[i-1]);
        } System.out.println(cnt[i-1]) System.out.println System.out System System.out cnt[i-1] cnt i-1 i 1 {
            System.out.println(cnt[i-1] + n - num[i-1]);
        } System.out.println(cnt[i-1] + n - num[i-1]) System.out.println System.out System System.out cnt[i-1] + n - num[i-1] cnt[i-1] + n cnt[i-1] cnt i-1 i 1 n num[i-1] num i-1 i 1 String[] args args static int digitCnt(int n) {
        int cnt = 0;
        while (n >= 10) {
            n /= 10;
            cnt++;
        }

        return cnt+1;
    } digitCnt {
        int cnt = 0;
        while (n >= 10) {
            n /= 10;
            cnt++;
        }

        return cnt+1;
    } int cnt = 0; cnt 0 while (n >= 10) {
            n /= 10;
            cnt++;
        } n >= 10 n 10 {
            n /= 10;
            cnt++;
        } n /= 10 n 10 cnt++ cnt return cnt+1; cnt+1 cnt 1 int n n public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        int[] num = {0, 10, 100, 1000, 10000, 100000};
        int[] cnt = {0, 9, 10, 909, 910, 90909};

        int i = digitCnt(n);
        if (i % 2 == 0) {
            System.out.println(cnt[i-1]);
        } else {
            System.out.println(cnt[i-1] + n - num[i-1]);
        }
    }

    static int digitCnt(int n) {
        int cnt = 0;
        while (n >= 10) {
            n /= 10;
            cnt++;
        }

        return cnt+1;
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        int[] num = {0, 10, 100, 1000, 10000, 100000};
        int[] cnt = {0, 9, 10, 909, 910, 90909};

        int i = digitCnt(n);
        if (i % 2 == 0) {
            System.out.println(cnt[i-1]);
        } else {
            System.out.println(cnt[i-1] + n - num[i-1]);
        }
    }

    static int digitCnt(int n) {
        int cnt = 0;
        while (n >= 10) {
            n /= 10;
            cnt++;
        }

        return cnt+1;
    }
} Main
import java.util.Scanner;

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

        int n = sc.nextInt();

        int[] num = {0, 10, 100, 1000, 10000, 100000};
        int[] cnt = {0, 9, 10, 909, 910, 90909};

        int i = digitCnt(n);
        if (i % 2 == 0) {
            System.out.println(cnt[i-1]);
        } else {
            System.out.println(cnt[i-1] + n - num[i-1]);
        }
    }

    static int digitCnt(int n) {
        int cnt = 0;
        while (n >= 10) {
            n /= 10;
            cnt++;
        }

        return cnt+1;
    }
}