import java.util.*;
import java.lang.*;
import java.io.*;
 
class A
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner in = new Scanner(System.in);
		int n, m, minus = 1;
		n = in.nextInt();
		m = 1;
		for  (int a = 1; n> 0; a *= 10) {
		  for  (int b = a; b < a*10; b++) {
			if ((n - minus) < 0) {
			  System.out.print(0);
			  return;
			}
			if ((n - minus) == 0) {
				System.out.print(m);
			  return;
			}
			n-= minus;
			m++;
		  }
		  minus++;
		}
		in.close();
		return;
	}
}