Please answer a question about the following article about Kanye West:

Fresh off spending the previous year touring the world with U2 on their Vertigo Tour, West felt inspired to compose anthemic rap songs that could operate more efficiently in large arenas. To this end, West incorporated the synthesizer into his hip-hop production, utilized slower tempos, and experimented with electronic music and influenced by music of the 1980s. In addition to U2, West drew musical inspiration from arena rock bands such as The Rolling Stones and Led Zeppelin in terms of melody and chord progression. To make his next effort, the third in a planned tetralogy of education-themed studio albums, more introspective and personal in lyricism, West listened to folk and country singer-songwriters Bob Dylan and Johnny Cash in hopes of developing methods to augment his wordplay and storytelling ability.

Who did Kanye study in order to develop his lyrical skills?
Bob Dylan and Johnny Cash