First, let's introduce two new concepts: **single-digit** and **multi-digit** numbers.

**Definition. A single-digit number is a number that consists of one digit. **For example, the following numbers are single digits:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9

The word "single digits" speaks for itself. One-digit means it consists of a single digit (a digit is sometimes called a sign).

**Definition. A multi-digit number is a number that consists of two or more digits.** For example, the following digits are multi-digit numbers:

10, 11, 15, 255, 350, 1000, 12500

There are an infinite number of multi-digit numbers. They cannot be counted. In addition, they are divided into the following types:

- two-digit numbers that consist of two digits (e.g., 25);
- three-digit numbers that consist of three digits (for example, 563);
- four-digit numbers that consist of four digits (e.g., 1400)

and so on, depending on how many digits are in the number.

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