Transit Visa Free - Two Stops in China

Hi, I am a US citizen flying by the first airline to Beijing with a stop in Shanghai. From Beijing, I am flying with another airline to Vladivostok. The stop in Shanghai is 11 hours. The flight to Beijing is 2 hours. I leave Beijing 36 hours after arriva in Beijingl. Am I covered under the 72-hour Visa Free Transit or my total stay cannot exceed 24 hours? Many thanks.