import java.util.Scanner;

public class Kaibun {

	public static void main(String[] args) throws InterruptedException {
		Scanner s = new Scanner(System.in);
		long t = (long)Math.pow(10, 9) +1 , N = s.nextLong();
		s.close();
		int end = (int)(N/t);
		long count = 0, stock = 0;
		int keta = Integer.toString(end).length();
		
		if(keta == 1){
			count = end;
		}else{
			stock = 9;
			for(int i = 1;i < keta;i++){
				count += stock;
				if(i%2 == 0){
					stock *= 10;
				}
			}
			int k = (int)Math.pow(10, keta/2);
			count += ((end - (int)Math.pow(10,keta-1))/k+1);
			
			StringBuilder str = new StringBuilder(Integer.toString(end/k));
			String str2 = str.toString();
			if(keta%2 == 1){
				str.deleteCharAt(str.length()-1);
			}
			str2 = str2.concat(str.reverse().toString());
			if(Integer.parseInt(str2) > end){
				count--;
			}
		}
		System.out.println(count);
	}
}