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 Pascal (FPC 2.6.2) (judge verdict: Accepted)

var
	n,i,sum:integer;
    
begin
	sum := 0;
	read(n);
    
   for i := 1 to n do begin
    	
        begin
        	if i <= 9 then
            	 Inc(sum);
        end;
        
        begin
        	if (i >= 100) and (i <= 999) then
            	Inc(sum);
        end;
        
         begin
        	if (i >= 10000) and (i <= 99999) then
            	Inc(sum);
        end;
        
    end;
    
    writeln(sum);

end.