project euler problem 412

Gnomon numbering

This problem is relatively easy with the help of google. 

I did some search about the problem and find wikipedia and mathworld provide some very useful information. I do not want to give too much hint since the search takes little effort. After I understood what needs to be computed, the rest is really simple since 76543217 is a prime.

It takes 0.35 second to get the correct answer.