Because the first grade is the first to know natural numbers. We use 1, 2, 3, 4, 5 ... When calculating an object, it is called a natural number. 1 is the unit of natural number and the smallest natural number, but there is no largest natural number.

Adding a negative sign before a natural number is called a negative integer. The unit of negative integer is-1, and-1 is the largest negative integer, but there is no smallest negative integer.

Zero is neither a natural number nor a negative integer. In notation, it means that a bit is empty or a neutral number between positive and negative numbers.