import java.util.Scanner;
public class Main{

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		
		int n = stdIn.nextInt();
		int cnt = 0;
		for(int i = 1; i <=n; i++) {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		}
		System.out.println(cnt);
		
		
	}

} import java.util.Scanner; Scanner java.util public class Main{

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		
		int n = stdIn.nextInt();
		int cnt = 0;
		for(int i = 1; i <=n; i++) {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		}
		System.out.println(cnt);
		
		
	}

} Main public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		
		int n = stdIn.nextInt();
		int cnt = 0;
		for(int i = 1; i <=n; i++) {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		}
		System.out.println(cnt);
		
		
	} main {
		Scanner stdIn = new Scanner(System.in);
		
		int n = stdIn.nextInt();
		int cnt = 0;
		for(int i = 1; i <=n; i++) {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		}
		System.out.println(cnt);
		
		
	} Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) int n = stdIn.nextInt(); n stdIn.nextInt() stdIn.nextInt stdIn int cnt = 0; cnt 0 for(int i = 1; i <=n; i++) {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		} int i = 1; int i = 1; i 1 i <=n i n i++ i++ i {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		} {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		} int nn = String.valueOf(i).length(); nn String.valueOf(i).length() String.valueOf(i).length String.valueOf(i) String.valueOf String i if(nn % 2 == 1) {
				cnt++;
			} nn % 2 == 1 nn % 2 nn 2 1 {
				cnt++;
			} cnt++ cnt System.out.println(cnt) System.out.println System.out System System.out cnt String[] args args public class Main{

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		
		int n = stdIn.nextInt();
		int cnt = 0;
		for(int i = 1; i <=n; i++) {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		}
		System.out.println(cnt);
		
		
	}

} public class Main{

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		
		int n = stdIn.nextInt();
		int cnt = 0;
		for(int i = 1; i <=n; i++) {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		}
		System.out.println(cnt);
		
		
	}

} Main
import java.util.Scanner;
public class Main{

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		
		int n = stdIn.nextInt();
		int cnt = 0;
		for(int i = 1; i <=n; i++) {
			int nn = String.valueOf(i).length();
			if(nn % 2 == 1) {
				cnt++;
			}
		}
		System.out.println(cnt);
		
		
	}

}
