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++) {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				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++) {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				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++) {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				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++) {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				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++) {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				ans++;
			}
		} int i=1; int i=1; i 1 i<=n i n i++ i++ i {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				ans++;
			}
		} {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				ans++;
			}
		} int cnt=0; cnt 0 int num=i; num i while(num>0) {
				cnt++;
				num/=10;
			} num>0 num 0 {
				cnt++;
				num/=10;
			} cnt++ cnt num/=10 num 10 if(cnt%2!=0) {
				ans++;
			} cnt%2!=0 cnt%2 cnt 2 0 {
				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++) {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				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++) {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				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++) {
			int cnt=0;
			int num=i;
			while(num>0) {
				cnt++;
				num/=10;
			}
			if(cnt%2!=0) {
				ans++;
			}
		}
		System.out.println(ans);
	}
}