Algorithms – Karatsuba multiplication rule to divide a number into two parts

In the Karatsuba algorithm to multiply two numbers, we divide each number into two. For example:

x= 1234
y= 2456

Then a = 12, b = 34, c = 24, d = 56

What happens if the digits in each number are not even or equal? What is the rule by dividing it into two parts?

Example:

 x = 12345
 y = 2478

or

 x = 12456778
 y = 241

Please help.