A power line is to be constructed from a power station at point A to an island at point C, which is 1 mi directly out in the water from a point B on the shore. Point B is 8 mi downshore from the power station at A. It costs $2800 per mile to lay the power line under water and $2000 per mile to lay the line under ground. At what point S downshore from A should the line come to the shore in order to minimize cost?