A group of k villagers must cross, the Sarawak river, which is a wide and deep river. There is no bridge in sight. They notice two 10-year-old boys playing in a rowboat at the shore. The boat is so tiny and it can hold only two boys or one villager. How can the villagers can get across the river and leave the boys in joint possession of the boat? How many times the boat pass from shore to shore?