(define (solve a b c d)
  (let ([n (max 0 (- (min b d) (max a c) -1))]
        [ab (- b a -1)]
        [cd (- d c -1)])
    (+ (* n (- cd 1)) (* (- ab n) cd))))

(define (main args)
  (let* ([a (read)]
         [b (read)]
         [c (read)]
         [d (read)])
    (print (solve a b c d)))
  0)