I didn't found how to evaluate the result exactly with a close
expression, but I just count all the results with loops and a
recursion.
For some of an input data it took more than 2 secs., therefore I ran
my program in offline for all possible input values.
(Thanks that there were not so many.)
The program running took a couple of minutes, then I collected all
the found results as an array and submitted the program that just
looked for the appropriate entry in the array and returned the right
answer!!!