(defun frog-river-one (x a)
  (let ((existing (make-hash-table)))
    (loop for k from 0
          for i in a
          do (when (and (not (gethash i existing)) (<= i x))
               (setf (gethash i existing) i)
               (when (= (hash-table-count existing) x)
                 (return-from frog-river-one k))))
    -1))