(define (solve a b s)
    (let ((da (abs (- a s))) (db (abs (- b s))))
        (cond
            ((or (<= da db) (= s 1)) (+ da s))
            ((not (= a 0)) (+ db a (min (- s 1) k)))
            (else (+ db s 1)))))

(let* ((a (read)) (b (read)) (s (read)))
    (print (solve a b s)))