All Substrings

You are given two strings $S$ and $R$. You may reorder the characters in the string $R$ in any way; let's denote the resulting string by $R_r$. This reordered string is *valid* if it contains all substrings of $S$, i.e. for each substring $s$ of $S$ (including $S$ itself), $R_r$ contains $s$ as a substring. Find the lexicographically smallest valid reordered string or determine that there is no s