Passage: Given a 6-bit input, the 4-bit output is found by selecting the row using the outer two bits (the first and last bits), and the column using the inner four bits. For example, an input "011011" has outer bits "01" and inner bits "1101"; the corresponding output would be "1001".The 8 S-boxes of DES were the subject of intense study for many years out of a concern that a backdoor — a vulnerability known only to its designers — might have been planted in the cipher. The S-box design criteria were eventually published (in Coppersmith 1994) after the public rediscovery of differential cryptanalysis, showing that they had been carefully tuned to increase resistance against this specific attack. Biham and Shamir found that even small modifications to an S-box could significantly weaken DES.There has been a great deal of research into the design of good S-boxes, and much more is understood about their use in block ciphers than when DES was released.Any S-box where each output bit is produced by a bent function of the input bits, and where any linear combination of the output bits is also a bent function of the input bits, is a perfect S-box.

Question: How do you find the 4-bit output?
selecting the row using the outer two bits (the first and last bits), and the column using the inner four bit