I was studying the designation of names to refrigerants 

I got the following basic formula :

R - (m - 1) ( n + 1) ( o )

where m = number of carbon atoms in the refrigerant 
      n = number of hydrogen atoms in the refrigerant 
      o = number of fluorine atoms in the refrigerant

so,
R - 134a has:

1 + 1 = 2 carbon atoms

3 - 1 = 2 hydrogen atoms

4 fluorine atoms

now, **what is 'a'?**