Problem with page numbers in Footers

The page numbers in footers of my word document are incorrect. I have selected “Different First page” in Page Setup. But the page numbers from second page onwards are totally mismatching. It shows page no. 20 on second page itself. It starts counting page numbers from 20 onwards. At the end of the document, page x is bigger than page y (i.e. page 50 of 47). Can anyone help me out to rectify the page numbers in Footer section?