finding recurrence for domino tiling ?

I am trying to solve this problem on spoj . By using tutorial in this Link I am able to figure out this recurrence.

*****    AA***    AA***    AA***    A****
***** =  AA***  + A****  + AA***  + A****
*****    AA***    A****    A****    AA***
*****    AA***    AA***    A****    AA***

f(n) = f(n-2) + h(n-1) + g(n-1) + g(n-1).
But I can’t understand how to solve the recurrence for h(n-1) and g(n-1).

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