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 Perl (v5.18.2) (judge verdict: Accepted)

($N) = split(/\s+/, <>);

$n = 0;

if ($N < 10) {
    # 1..$N: $N
    $n = $N;
} elsif ($N < 100) {
    # 1..9: 9
    # 10..99: 0
    $n = 9;
} elsif ($N < 1000) {
    # 1..9: 9
    # 10..99: 0
    # 100..$N: $N + 1
    $n = 9 + ($N - 100 + 1);
} elsif ($N < 10000) {
    # 1..9: 9
    # 10..99: 0
    # 100..999: 900
    # 1000..$N: 0
    $n = 9 + 900;
} elsif ($N < 100000) {
    # 1..9: 9
    # 10..99: 0
    # 100..999: 900
    # 1000..9999: 0
    # 10000..$N: $N - 100000 + 1
    $n = 9 + 900 + ($N - 10000 + 1);
} elsif ($N == 100000) {
    $n = 9 + 900 + 90000;
}

print "$n\n";