The first digit can not be 0 and so for the first digit we have 9 options (from 1 to 9). Now we have to choose the next digit in such a way that it is not the same as the previous one. Thereofore on choosing the next digit we can have 9 options as well, (because we can choose any number from 0 to 9 except the previous one). Therefore, each of the remaining 19 digits can only have 9 options. Therefore there are 9×199 numbers are available of the given type.