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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws Exception {
        Main main = new Main();
        main.solve();
    }
    public void solve() throws Exception {
        Scanner scan = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        int N = scan.nextInt();
        String d = Integer.toString(N);
        int length = d.length();
        long answer = 0;
        if (length % 2 == 0) {
            long base = 1;
            for (int i = 1; i < length; i += 2) {
                answer += 9 * base;
                base *= 100;
            }
        } else {
            long base = 1;
            for (int i = 1; i < length - 1; i += 2) {
                answer += 9 * base;
                base *= 100;
            }
            answer += N - (long) Math.pow(10, length-1) + 1;
        }
        System.out.println(answer);
    }
}