Restricting the processing of line items using badi in me51n t-code

sir,

While creating PR using T Code ME51N a check should be made to restrict processing if the line items contain material of type ‘ZERS’

Within this badi, material lines need to be checked from table MARA . If the material type fetched from MARA is ‘ZERS’ , check account assignment within parameter,If the a/c assignment is ‘K’ system should throw an error message saying ‘Creation of PR for ‘ZERS’ material with A/C assignment ‘K’ is not permitted. ’

pls suggest the exact badi, where i can incorporate my code for ME51N (Purchase Requistion).