import java.util.*;
 
public class Main {
	
    public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int count = 0;
    	for (int i = 1; i <= N; i++) {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	}
    	System.out.println(count);
    }
} import java.util.*; util.* java public class Main {
	
    public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int count = 0;
    	for (int i = 1; i <= N; i++) {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	}
    	System.out.println(count);
    }
} Main public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int count = 0;
    	for (int i = 1; i <= N; i++) {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	}
    	System.out.println(count);
    } main {
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int count = 0;
    	for (int i = 1; i <= N; i++) {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	}
    	System.out.println(count);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int count = 0; count 0 for (int i = 1; i <= N; i++) {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	} int i = 1; int i = 1; i 1 i <= N i N i++ i++ i {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	} {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	} if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		} Integer.toString(i).length()%2 != 0 Integer.toString(i).length()%2 Integer.toString(i).length() Integer.toString(i).length Integer.toString(i) Integer.toString Integer i 2 0 {
    			count++;
    		} count++ count System.out.println(count) System.out.println System.out System System.out count String[] args args public class Main {
	
    public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int count = 0;
    	for (int i = 1; i <= N; i++) {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	}
    	System.out.println(count);
    }
} public class Main {
	
    public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int count = 0;
    	for (int i = 1; i <= N; i++) {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	}
    	System.out.println(count);
    }
} Main
import java.util.*;
 
public class Main {
	
    public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int count = 0;
    	for (int i = 1; i <= N; i++) {
    		if (Integer.toString(i).length()%2 != 0) {
    			count++;
    		}
    	}
    	System.out.println(count);
    }
}