Dominoes

The X's at the bottoms of the dominoes mean that these areas are not visible, so all you know is that one of the dominoes has a five-spot and the other has six-spot.

If the probality that you can form an end-to-end chain of all 28 dominoes-with the two decipted dominoes in first and last position-subject to the usual rule that the number on the right of any dominoe in the chain always equals the number on the left of the next domino is $\dfrac{a}{b}$ , where $\text{gcd}(a, b)=1$ , then what is $a+b$ ?