import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int s = 0 ; 
		for(int i = 1 ; i <= n ; i++)
		{
			if((i*i)%10 == i || (i*i)%100 == i || (i*i)%1000 == i || (i*i)%10000 == i ||(i*i)%100000 == i)
			{
				s++;
			}
		}
		System.out.println(s);
  	}
 }
/**************************************************************
	Problem: 1729
	User: admin
	Language: Java
	Result: Accepted
	Time:2208 ms
	Memory:40328 kb
****************************************************************/