What minimum number of stones must be added

Question: Native Americans boiled water by adding very hot stones to a leak-tight water vessel. What minimum number of 1.0 kg stones at 530 °C must be added to a vessel holding 7.0 kg of 20 °C water to bring the water to a boil? Use 800 J/(kg.K) for the specific heat of the stones and 4190 J/(kg.K) for the specific heat of water.