Formulae by the criss-cross method

You will write a function that makes a formula from two parts and their valencies by the criss-cross method, simplifies it, and puts a group of atoms in brackets when there is more than one of it. The program then writes six formulas.

The idea. Cross over the valencies, divide both numbers by their highest common factor, leave out a $1$, and bracket a group such as OH or $\text{SO}_4$ that appears more than once.

Your tasks.

1. Cross over: set n1 to the valency of the second part and n2 to the valency of the first.
2. Simplify: divide both by g, their highest common factor.
3. In part, put a group in brackets when its number is more than $1$.