Different counting units are arranged in a certain order, and their positions are called numbers. The number of digits in an integer increases gradually from right to left: the first digit is a single digit, the second digit is a ten digit, the third digit is a hundred digit, the fourth digit is a thousand digit, the fifth digit is a million digit, the sixth digit is a hundred thousand digit, the seventh digit is a million digit, and the eighth digit is a million digit ... What's the difference between a counting unit and a number? Every number should have a name so that we can call it, that is, we can read it. Take natural numbers as an example. There are infinite natural numbers. If every natural number is read with an independent name, it is very inconvenient and impossible. In order to solve this problem, people have created a counting system, which is the decimal counting method we are using now. The decimal counting method is characterized by "full 10 into one". In other words, every 10 unit forms a higher unit adjacent to it. That is, 10 is called "ten", 10 is called "hundred", 10 is called "thousand" and 10 is called "ten thousand". One (a), ten, hundred, thousand, ten thousand, one hundred thousand, one million (trillion), ten million, one billion, ten billion, one trillion, one trillion, ... are all units of counting. Numbers mean that when writing numbers, the numbers are arranged side by side in a line, and each number occupies a position. These positions are called numbers. Starting from the right end, the first place is the unit, the second place is the tenth place, the third place is the hundred places, the fourth place is the thousand places, and the fifth place is the ten thousand places, and so on. This shows that the concept of counting unit and number is different. However, their relationship is very close. This is because the counting unit is one, the counting unit is ten, the counting unit is hundred, the counting unit is thousand, and the counting unit is ten thousand. For example: 8475, where "8" means eight thousands in thousands, "4" means four hundreds in hundreds, "7" means seven tens in tens, and "5" means five ones in tens.