N = int(eval(input()))



ans = 0



for i in range(1, N+1):

    if len(str(i))%2==1:

        ans += 1



print(ans)
import sys





def rec(length):

    if length == 1:

        return 9

    else:

        return 9 * 10**(length - 1) + rec(length - 2)





n = eval(input())

length = len(n)



if length == 1:

    print(n)

    sys.exit()



if length % 2 == 0:

    ans = rec(length - 1)

else:

    ans = rec(length - 2) + int(n) - int("1" + "0" * (length - 1)) + 1



print(ans)
N = int(eval(input()))
ans = 0
for i in range(1, N + 1):
    if len(str(i)) % 2 == 1:
        ans += 1
print(ans)

import sys


def rec(length):
    if length == 1:
        return 9
    else:
        return 9 * 10 ** (length - 1) + rec(length - 2)


n = eval(input())
length = len(n)
if length == 1:
    print(n)
    sys.exit()
if length % 2 == 0:
    ans = rec(length - 1)
else:
    ans = rec(length - 2) + int(n) - int("1" + "0" * (length - 1)) + 1
print(ans)

-N = int(eval(input())) -ans = 0 -for i in range(1, N + 1): -    if len(str(i)) % 2 == 1: -        ans += 1 +import sys + + +def rec(length): +    if length == 1: +        return 9 +    else: +        return 9 * 10 ** (length - 1) + rec(length - 2) + + +n = eval(input()) +length = len(n) +if length == 1: +    print(n) +    sys.exit() +if length % 2 == 0: +    ans = rec(length - 1) +else: +    ans = rec(length - 2) + int(n) - int("1" + "0" * (length - 1)) + 1