import core.bitop, std.algorithm, std.ascii, std.bigint, std.conv, std.math,
    std.functional, std.numeric, std.range, std.stdio, std.string, std.random,
    std.typecons, std.container, std.format;

// dfmt off
T lread(T = long)(){return readln.chomp.to!T();}
T[] aryread(T = long)(){return readln.split.to!(T[])();}
void scan(TList...)(ref TList Args){auto line = readln.split();
foreach (i, T; TList){T val = line[i].to!(T);Args[i] = val;}}
alias sread = () => readln.chomp();enum MOD = 10 ^^ 9 + 7;
// dfmt on

long f(long n)
{
    long c = 0;
    while (n != 0)
    {
        n /= 10;
        c++;
    }
    return c;
}

void main()
{
    long N = lread();
    long cnt;
    foreach (x; 1 .. N + 1)
    {
        if (f(x) & 1)
            cnt++;
    }
    writeln(cnt);
}
