given a question and a context, extract the answer or return INVALID if the answer is not in the context.
question: what is an RNN?
context:
 * doc title: AI models
 * content: Unleash uses different models to achieve efficient and reliable search, even in unstable territories. We use RNNs, we use GPT, we also partner with AI-21. By combining different models and doing validations, we achieve stability that  is so missing in this field.