OMM 1994 Problem 4

A capricious mathematician writes a book with pages numbered from $2$ to $400$ . The pages are to be read in the following order. Take the last unread page ( $400$ ), then read (in the usual order) all pages which are not relatively prime to it and which have not been read before. Repeat until all pages are read. So, the order would be $2, 4, 5, ... , 400, 3, 7, 9, ... , 399, ...$ . What is the last page to be read?