Pentagonal numbers are generated by the formula, `Pn=n(3n-1)/2`. The first ten pentagonal numbers are:

> 1, 5, 12, 22, 35, 51, 70, 92, 117, 145, ...

It can be seen that `P4 + P7 = 22 + 70 = 92 = P8`. However, their difference, `70 - 22 = 48`, is not pentagonal.

Find the pair of pentagonal numbers, Pj and Pk, for which their sum and difference are pentagonal and `D = | Pk - Pj |` is minimised; what is the value of D?
