Kata

If a chessboard were to have wheat placed upon each square such that one grain were placed on the first square, two on the second, four on the third, and so on (doubling the number of grains on each subsequent square), how many grains of wheat would be on the chessboard at the finish?

We suppose that our chessboard is nxn (n rows and n columns) Write a function grains(n) which computes the number of grains on the chessboard.