assuming the list is just random numbers the quickest method would be as follows.

Let your list be

$a_{0,i},~i=1,N$ (the reason for the zero will become apparent)

now form a new list

$a_{1,1}=$a_{0,1}+a_{0,2}$

$a_{1,2}=$a_{0,3}+a_{0,4}$

.

.

$a_{1,k}=a_{0,2k-1}+a_{0,2k}$

i.e. we add adjacent pairs of numbers from the 0th list to come up with the 1st list.

Repeat to form $a_{2,k}, a_{3,k}$, etc. until there is only a single number left.

This number is the sum of all the numbers on your list.

Subtract this from your constant.

To make this a bit clearer consider

$a_0=\{ 1, 3, -2, 7, 13, -5, -1, 6\}$

we form

$a_1=\{ (1+3), (-2+7), (13-5),(-1+6) \} =\{ 4, 5, 8, 5 \}$

$a_2 = \{ (4+5), (8+5) \}=\{ 9, 13 \}$

$a_3 = \{ (9+13) \} = \{ 22 \}$

so subtract 22 from your constant.