How to create flat discount pricing rules

Scenario: if any customer buys item code1 and item code2 product then he get flat 20% discount on total amount.
Example: item_code1 price 100$ item_code2 price 200$ . Assume, customer buy both product in single qty. then total amount 300$ after applying 20% discount on 300$ the paid amount equal 240$.

How can i do that by pricing rules
ERP Version: 12.5.2