import java.util.Scanner;

public class Main {
	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 || (100<=i && i<1000) || (10000<=i && i<100000)) {
				ans++;
			}
		}
		System.out.println(ans);
	}
} 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 ans=0;

		for(int i=1;i<=n;i++) {
			if(i<10 || (100<=i && i<1000) || (10000<=i && i<100000)) {
				ans++;
			}
		}
		System.out.println(ans);
	}
} Main 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 || (100<=i && i<1000) || (10000<=i && 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 || (100<=i && i<1000) || (10000<=i && 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 || (100<=i && i<1000) || (10000<=i && i<100000)) {
				ans++;
			}
		} int i=1; int i=1; i 1 i<=n i n i++ i++ i {
			if(i<10 || (100<=i && i<1000) || (10000<=i && i<100000)) {
				ans++;
			}
		} {
			if(i<10 || (100<=i && i<1000) || (10000<=i && i<100000)) {
				ans++;
			}
		} if(i<10 || (100<=i && i<1000) || (10000<=i && i<100000)) {
				ans++;
			} i<10 || (100<=i && i<1000) || (10000<=i && i<100000) i<10 || (100<=i && i<1000) || (10000<=i && i<100000) i<10 i 10 (100<=i && i<1000) 100<=i 100 i i<1000 i 1000 (10000<=i && i<100000) 10000<=i 10000 i i<100000 i 100000 {
				ans++;
			} ans++ ans System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
	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 || (100<=i && i<1000) || (10000<=i && i<100000)) {
				ans++;
			}
		}
		System.out.println(ans);
	}
} public class Main {
	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 || (100<=i && i<1000) || (10000<=i && i<100000)) {
				ans++;
			}
		}
		System.out.println(ans);
	}
} 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 ans=0;

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