Omnicom is buying Interpublic Group in a stock-for-stock deal that will create an advertising powerhouse with combined annual revenue of almost $26 billion. The companies have had a hand in iconic marketing campaigns like “Got Milk” for the California Milk Processor Board, “Priceless” for Mastercard, “Because I’m Worth It” for L’Oreal and “Think Different” for Apple. The combined company will be worth more than $30 billion. Shares of Interpublic jumped more than 15% before the opening bell Monday, while Omnicom’s stock fell more than 2%.