solve.rkt

#lang racket

(define input (map (compose1 string->number string)
                   (string->list (read-line))))

(define (solve input [offset 1])
  (for/sum ([a input]
            [b (sequence-tail (in-cycle input) offset)]
            #:when (= a b)) b))

(solve input)
(solve input (/ (length input) 2))