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The total of an odd number of consecutive numbers is always the middle number multiplied by however many numbers are listed (midnumber x amountofnumbers).
If we have 1,2,3,4,5:

We have 5 numbers, and the middle number is 3, so the total must be 15.
This is because the two numbers behind it are both the middle number -1 and -2, and the two numbers after it are both the middle number +1 and +2. These cancel each other out, so the list is essentially the same as 3,3,3,3,3.