import java.util.Scanner;
 
public class Main {
 
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int res = 0;

		//　桁数が奇数の数を数える
		for(int i = 1; i <= n; i++) {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		}
		
		System.out.println(res);
		sc.close();
	}
} 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 res = 0;

		//　桁数が奇数の数を数える
		for(int i = 1; i <= n; i++) {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		}
		
		System.out.println(res);
		sc.close();
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int res = 0;

		//　桁数が奇数の数を数える
		for(int i = 1; i <= n; i++) {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		}
		
		System.out.println(res);
		sc.close();
	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int res = 0;

		//　桁数が奇数の数を数える
		for(int i = 1; i <= n; i++) {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		}
		
		System.out.println(res);
		sc.close();
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int res = 0; res 0 for(int i = 1; i <= n; i++) {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		} int i = 1; int i = 1; i 1 i <= n i n i++ i++ i {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		} {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		} if(Integer.toString(i).length() % 2 == 1) {
				res++;
			} Integer.toString(i).length() % 2 == 1 Integer.toString(i).length() % 2 Integer.toString(i).length() Integer.toString(i).length Integer.toString(i) Integer.toString Integer i 2 1 {
				res++;
			} res++ res System.out.println(res) System.out.println System.out System System.out res sc.close() sc.close sc String[] args args public class Main {
 
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int res = 0;

		//　桁数が奇数の数を数える
		for(int i = 1; i <= n; i++) {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		}
		
		System.out.println(res);
		sc.close();
	}
} public class Main {
 
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int res = 0;

		//　桁数が奇数の数を数える
		for(int i = 1; i <= n; i++) {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		}
		
		System.out.println(res);
		sc.close();
	}
} 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 res = 0;

		//　桁数が奇数の数を数える
		for(int i = 1; i <= n; i++) {
			if(Integer.toString(i).length() % 2 == 1) {
				res++;
			}
		}
		
		System.out.println(res);
		sc.close();
	}
}