In a famous story, a wise man in India requests a reward promised by his king. The wise man shows the king a chessboard having 64 squares. "Put 1 grain of rice on the first square, 2 grains on the second square, 4 on the third square, 8 on the fourth square, and so on, doubling the number of grains each time, until all 64 squares are filled with the right number of grains. That will be my reward." How many rice grains must be given in total ?