How many four-digit numbers are there whose decimal notation contains not more than two distinct digits?

Explanation

To find the number of four-digit numbers whose decimal notation contains not more than two distinct digits, we can break the problem down into cases.

### Step-by-Step Solution

**Step 1: Count the case where all four digits are the same.**
- The four-digit numbers can only be formed using the digits 1 to 9 (since a four-digit number cannot start with 0).
- The possible numbers are 1111, 2222, ..., 9999.
- Thus, there are 9 such numbers (one for each digit from 1 to 9).
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