How To Give Primary Key Increment And Decrement Automatically

I want make one database about office register. I gave primary key for auto increment serial number . That is working fine. Now problem is some times I want delete client name in between table .That is maybe first row or 5th row or anywhere . at the time the row deleted which is i was selected but the primary key is not update . for example I want delete 8th row . after deleting I not seen 8 in primary key place i have seen only ....,5,6,7,9,10...... etc. how to give auto increment and decrement in the primary key......