I want to create macro that would Enter ID number in column A if value is
entered in column B. For example i have following

How do i execution of code if any value is entered in certain column' cell ?
For example i want to enter value in column A if ANY value is entered in
column B

Example:
A B C
1 1 aaa
2 2 bbb
3

I want to create code that would enter next number in column A if i enter
any value in column B.
Value A2+1 should be entered in cell B3 if i enter value "s" in cell B3.
Like this

A B C
1 1 aaa
2 2 bbb
3 3 s