On page 46 of these lecture notes, it seems to say that if we have a Feistel cipher, and plaintexts $(L_0, R_0)$ and $(L_0^*, R_0^*)$ with corresponding encryptions, then we can determine the key? But isn't this not the case by Luby-Rackoff? I'm not entirely sure what the slides are even saying. They say that we can compute $R_3 \oplus R_3^*$, but so what? How does this help determine the key?