from itertools import *
from bisect import *
#from math import *
from collections import *
from heapq import *
from random import *
from decimal import *
import numpy as np
import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def MI1(): return map(int1, sys.stdin.readline().split())
def MF(): return map(float, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LF(): return list(map(float, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
dij = [(1, 0), (0, 1), (-1, 0), (0, -1)]

def main():
    s=input()
    n=int(s)
    if n<10**9+1:
        print(0)
        exit()
    k=len(s)-9
    ans=0
    for i in range(1,k):
        ans+=10**((i+1)//2)-1
    ans+=int(s[:(k+1)//2])-(10**((k-1)//2)-1)
    sym=int(s[:(k+1)//2]+s[:k//2][::-1])
    ls=int(s[:k])
    if ls<sym:ans-=1
    if ls==sym and int(s[-k:])<ls:ans-=1
    print(ans)

main()