/* -*- coding: utf-8 -*-
 *
 * 491.cc: No.491 10^9+1と回文 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

/* typedef */

typedef long long ll;

/* global variables */

/* subroutines */

int ts[10], es[10];

/* main */

int main() {
  ts[0] = 1;
  for (int i = 1; i <= 9; i++) ts[i] = ts[i - 1] * 10;
  es[0] = 1;
  es[1] = es[2] = 9;
  for (int i = 3; i <= 9; i++) es[i] = es[i - 2] * 10;
  //for (int i = 0; i <= 9; i++) printf("es[%d]=%d\n", i, es[i]);

  ll n;
  cin >> n;

  int m = n / 1000000000;
  if (m > n % 1000000000) m--;
  //printf("m=%lld\n", m);

  int l = 0;
  for (int mm = m; mm > 0; l++, mm /= 10);
  //printf("l=%d\n", l);

  int sum = 0;
  for (int i = 1; i < l; i++) sum += es[i];

  bool odd = ((l & 1) == 1);
  int l0 = (l + 1) / 2, l1 = l - l0;

  for (int a = ts[l0 - 1];; a++) {
    int c = a * ts[l1];
    for (int b = (l0 > l1) ? a / 10 : a, t = ts[l1 - 1];
	 b > 0; b /= 10, t /= 10)
      c += (b % 10) * t;
    if (c > m) break;
    sum++;
  }
  printf("%d\n", sum);
  return 0;
}