**Neko** is crazy at solving the algorithm problems recently. He sets a plan for himself:

The \(1^{st}\) day, he will get on 1 problem solved.

The \(2^{nd}\) day, he will get on 3 problems solved.

The \(3^{rd}\) day, he will get on 6 problems solved.

...

The \(n^{th}\) day, he will get on \(n(n+1)/2\) problems solved.

**Neko** wants to know how many problems he will solve after \(K\) days.