import java.util.*;

public class Main {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
int n = scan.nextInt();

Num(n);

}

static void Num(int n) {

int hanNum = 0;

int a[] = new int[3];

for (int i = 1; i < n; i++) {
if (i > 0 && i < 100) {
hanNum++;

} else if (i >= 1000)
break;
else {
a[0] = i / 100;
a[1] = (i % 100) / 10;
a[2] = i % 10;

if (a[0] - a[1] == a[1] - a[2])
hanNum++;
}

}
System.out.println(hanNum);

}
}

뭐가 틀린지 모르겠어요ㅜㅜ