How to express - 8+4-2+1 in sigma notation

It's pretty clear what the pattern is - 2^ (k-1) * (-1) ^ (k-1) would work backwards, but we have to work forwards to get lower in this case, so we could start at - 3 (as long as we put the k-1 in absolute value to make sure it's not 1/8), so we could write it as ∑ (k=1, goes to 4) 2^ (|k-4|) * (-1) ^ (k-4)

It doesn't matter for k-4 because 1/-1 and 1/1 are the same thing as - 1 and 1 respectively