For any language L, and for any k ≥ 0, let L k denote the language obtained by concatenating any k strings from L (note that L^0 = epsilon). Suppose L1 is a context-free language and R is a regular language over the same alphabet Σ of size at least 3. The smallest value of k for which (R \ L1) k ∪ (L1 \ R) is not necessarily context-free is