Karatsuba multiplication on numbers with odd length
Here, $B = 10$ and $n = 3$. The Karatsuba method works for any $m &lt; n$. With $m = 2$ :

$a = 2 * 10^2 + 34$
and
$b = 8 * 10^2 + 57$
; thus, $a_1 = 2$, $a_0 = 34$, $b_1 = 8$ and $b_0 = 57$.

It follows that $ab = z_2 B^{2m} + z_1 B^m + z_0$ where $z_2 = a_1 b_1 = 16$, $z_1 = a_1 b_0 + a_0 b_1 = 386$ and $z_0 = a_0 b_0 = 1938$.

Thus, $ab = 16 \cdot 10^{2*2} + 386 \cdot 10^2 +  1938 = 200538$.

I'm not sure about the step you mentioned, as it doesn't seem to give the correct result with the current decomposition ($m = 2$).