import core.bitop, std.bitmanip;
import core.checkedint;
import std.algorithm, std.functional;
import std.array, std.container;
import std.bigint;
import std.conv;
import std.math, std.numeric;
import std.range, std.range.interfaces;
import std.stdio, std.string;
import std.typecons;

void main()
{
    int n;
    readf("%s", &n);
    readln;
    
    int ans = 0;
    foreach (i; 1 .. n+1) {
        auto s = i.to!string;
        if (s.length % 2 == 1) { ++ans; }
    }
    
    ans.writeln;
}