How do you extract the last character from a non-empty string (DrRacket). Hope for assistance

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I'm fairly new to Scheme and I'm using DrRacket and I hope to get some much needed assistance.

I was first tasked to extract the first character from a non-empty string, which I was successful in doing so.

;;Signature: string-first: string -> string
;;Purpose: To extract the first character from a non-empty string
;;Tests
(check-expect (string-first "cookies") "c")
(check-expect (string-first "party") "p")
;;Definition: string-first
(define (string-first str)
(string-ith str 0))

Now, I am tasked with extracting the last character from a non-empty string. I cannot figure this out. My guess is that it involves "string-length" in combination with "string-ith."

Hopefully someone can help me out. Thank you in advance.