#!/usr/bin/env python3
# from typing import *

import sys
import io
import math
import collections
import decimal
import itertools
import bisect
import heapq


def input():
    return sys.stdin.readline()[:-1]


# sys.setrecursionlimit(1000000)

# _INPUT = """4000000000
# """
# sys.stdin = io.StringIO(_INPUT)

INF = 10**10

MOD = 1000000007


def solve(N):
    if N < 10**9+1:
        return 0
    N1 = N // (10**9)
    N2 = N % (10**9)

    d = len(str(N1))
    ans = 0
    for d1 in range(1, d):
        i = (d1+1)//2 - 1
        ans += 9 * (10 ** i)
    a = N1 // (10**(d//2))
    
    ans += int(str(int(str(a)[0])-1) + str(a)[1:])
    if d % 2 == 0:
        b = int(str(a) + str(a)[::-1])
    else:
        b = int(str(a) + str(a)[:-1][::-1])
    if b <= N2:
        ans += 1
    
    return ans


N = int(input())
print(solve(N))