Each single provides (2) buns and (1) patty. Each double provides (2) buns and (2) patties. Given (S) singles and (D) doubles, we will have (2*(S + D)) buns and (S + 2*D) patties.
A (K)-decker happens to require (K+1) buns and (K) patties. To know if we can build one, it suffices to check that both (2*(S + D) \ge K + 1) and (S + 2*D \ge K) hold true.