import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		if (N < 10) {
			System.out.print(N);
		} else if (N < 100) {
			System.out.print(9);
		} else if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		}
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		if (N < 10) {
			System.out.print(N);
		} else if (N < 100) {
			System.out.print(9);
		} else if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		}
	}
} Main public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		if (N < 10) {
			System.out.print(N);
		} else if (N < 100) {
			System.out.print(9);
		} else if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		}
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		if (N < 10) {
			System.out.print(N);
		} else if (N < 100) {
			System.out.print(9);
		} else if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc if (N < 10) {
			System.out.print(N);
		} else if (N < 100) {
			System.out.print(9);
		} else if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		} N < 10 N 10 {
			System.out.print(N);
		} System.out.print(N) System.out.print System.out System System.out N if (N < 100) {
			System.out.print(9);
		} else if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		} N < 100 N 100 {
			System.out.print(9);
		} System.out.print(9) System.out.print System.out System System.out 9 if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		} N < 1000 N 1000 {
			System.out.print(9 + (N - 99));
		} System.out.print(9 + (N - 99)) System.out.print System.out System System.out 9 + (N - 99) 9 (N - 99) N 99 if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		} N < 10000 N 10000 {
			System.out.print(909);
		} System.out.print(909) System.out.print System.out System System.out 909 if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		} N < 100000 N 100000 {
			System.out.print(909 + (N - 9999));
		} System.out.print(909 + (N - 9999)) System.out.print System.out System System.out 909 + (N - 9999) 909 (N - 9999) N 9999 {
			System.out.print(90909);
		} System.out.print(90909) System.out.print System.out System System.out 90909 String[] args args public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		if (N < 10) {
			System.out.print(N);
		} else if (N < 100) {
			System.out.print(9);
		} else if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		}
	}
} public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		if (N < 10) {
			System.out.print(N);
		} else if (N < 100) {
			System.out.print(9);
		} else if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		}
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		if (N < 10) {
			System.out.print(N);
		} else if (N < 100) {
			System.out.print(9);
		} else if (N < 1000) {
			System.out.print(9 + (N - 99));
		} else if (N < 10000) {
			System.out.print(909);
		} else if (N < 100000) {
			System.out.print(909 + (N - 9999));
		}else {
			System.out.print(90909);
		}
	}
}
