∑k=0k≤⌊n2⌋(nk)×(n−kk)\sum_{k=0}^{k\le \left\lfloor\dfrac{n}{2}\right\rfloor}\dbinom{n}{k}\times \dbinom{n-k}{k}∑k=0k≤⌊2n⌋(kn)×(kn−k)
