Berkshire Hathaway recently filed its second-quarter 13F form, revealing its stock holdings at the end of June.

The investment comes as UnitedHealth faces soaring medical costs, federal investigations, the fallout of the killing of a top executive and a cyberattack last year.

Berkshire Hathaway’s portfolio added six new stocks, including the three new secret stocks. Warren Buffett was a net seller of stocks for the eleventh straight quarter.

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Given Buffett's concern over the stock market, it makes sense for investors to buy the most conservative stocks in the Berkshire Hathaway portfolio.

However, Buffett has a keen eye for value and he never wants to overpay for a stock -- not even his own. Berkshire is currently trading at a price-to-sales ratio (P/S) of 2.5, which is a huge 25% premium to its 10-year average of 2.