#!/usr/bin/python

N = int(input())
top = N // 1000000001
count = 0

k = len(str(top))

for i in range(1, k):
    count += 9 * 10 ** ((i - 1) // 2)
keta = (k + 1) // 2
ue = top // 10 ** (k - keta)
sita = int(str(top)[::-1]) // 10 ** (k - keta)
count +=  ue - 10 ** (keta - 1) + 1
if ue > sita:
    count -= 1
print(count)