Thanks, Babaoriley!

I should have explained the formula better. There's 1 (= 2^0) grain on the first field, 2 (= 2^1) grains on the second field, 4 (= 2^2) on the third, and so on until 2^63 on the 64th field.

Thus the sum is: 2^0 + 2^1 + 2^2 + … + 2^63 = 2^64 - 1