var sum uint64
	for i := 1; i < 65; i++ {
		tmp, err := Square(i)
		if err != nil {
			return 0
		}
		sum += tmp

	// 2^63 + 2^62 + ... + 2^1 + 2^0
	// which means we have to flip all bits to 1 in an uint64
	var total uint64
	return ^total
}
// SumSquare returns the total number of grains to n-th square.
// SumSquare(64) == Total()
func SumSquare(n int) (uint64, error) {
	if n < 1 || n > 64 {
		return 0, fmt.Errorf("input must be positive and <= 64, got: %q", n)

	return sum

	var tmp uint64
	return ^tmp >> (64 - n), nil