Project Euler 025 BROKEN

025.scm

(define (fib n) 
  (let ((alpha (/  (+ 1 (sqrt 5)) 2) )
        (beta  (/  (- 1 (sqrt 5)) 2) )) 

        (- (* (/ 1 (sqrt 5)) (expt alpha n)) 
           (* (/ 1 (sqrt 5)) (expt beta n)))) )

(define hold (expt 10 999)) 
(define (problem025)
  (let loop ((mini 1) (maxi 10000))
    (let ((pibot (floor (/ (+ mini maxi) 2))))
        (cond ((>= mini maxi) mini) 
              ((< (fib pibot) hold) (loop mini pibot))
              (else (loop pibot maxi))  ))))