I recommend the first method of pairing 1 to 999, 2 to 998 and so on. Then, find how many pairs (49) becos there's only one 500 and the only way to add up to 1000 is another 500. Then, calculate the total by no. of pairs x 1000 + 500 + 1000 (at the end). The alternative method is just extra exploration using Arithmetic Progression series to get the answer. Hope this helps! ;))