I have read the related question(Difference between align and alignat environments), which had detailed answer but still don't know what the meaning of the
\alignat's argument. It is said in http://people.cs.uchicago.edu/~ivan/math/amsldoc.pdf that
This environment takes one argument, the number of “equation columns”: count the maximum number of
&s in any row, add 1 and and divide by 2.
What is the meaning of adding one in
& number and divide by two? I presumed the argument is given by the user, not automatically generated by the system?