An oil refinery is located on the north bank of a straight river that is [math] wide. A pipeline is to be constructed from the refinery to storage tanks located on the south bank of the river [math] east of the refinery. The cost of laying pipe is [math] over land to point [math] on the north bank and [math] under river to the tanks. What is the minimal cost to construct a pipeline from the refinery to the storage tanks?