Sorting X+Y (Pairwise Sums)

Given sorted or unsorted sets $X=\{x_1,\ldots,x_n\}$ and $Y=\{y_1,\ldots,y_n\}$ of real numbers, output the multiset $X+Y=\{x_i+y_j:1\leq i,j\leq n\}$ in nondecreasing order. Determine whether this can be done in $O(n^2)$ time in a standard real-comparison model.