Hi Rashid,
You can separate your article with a readmore hr tag, content above the readmore is your introtext and K2 readmore will appear after it.
Or you can go to K2 parameters configuration -> Advanced
and Set the Use one editor window for introtext & fulltext to NO. Now when adding items, you will be presented with 2 boxes, one for introtext and the other for fulltext instead of just one box.
K2 Rocks \m/