According to legends, the Shah of Persia was greatly please by the game of chess, and decided to reward it's inventor. Asked what reward he desires, the inventor of chess asked to be given grains of wheat for each square of a chess board: one grain for the first square, two grains for the second square, 4 grains for the third square, and so on, the amount of grain for each square being the double of the amount of the previous square. Write down the total reward, using the ΣΣ notation.