Problem:
Score : 200 points

Problem Statement

Given is an integer N. Find the number of positive integers less than or equal to N that have an odd number of digits (in base ten without leading zeros).

Constraints

1 \leq N \leq 10^5

Input

Input is given from Standard Input in the following format:

N

Output

Print the number of positive integers less than or equal to N that have an odd number of digits.

Sample Input 1

11

Sample Output 1

9

Among the positive integers less than or equal to 11, nine integers have an odd number of digits: 1, 2, \ldots, 9.

Sample Input 2

136

Sample Output 2

46

In addition to 1, 2, \ldots, 9, another 37 integers also have an odd number of digits: 100, 101, \ldots, 136.

Sample Input 3

100000

Sample Output 3

90909

Solution in Python (3.4.3) (judge verdict: Accepted)

# -*- coding utf-8 -*-
N = int(input())

ans = 0
if N < 10:
    ans = N
elif N < 100:
    ans = 9
elif N < 1000:
    ans += 9
    ans += max(0, N - 99)
elif N < 10000:
    ans += 9 + 900
elif N < 100000:
    ans += 9 + 900
    ans += max(0, N - 9999)
else:
    ans += 9 + 900 + 90000

print(ans)