(1). Because of 1-3, there are two numbers at this time, namely 1 and 3.
(2). Look at 1-3+5-7 again. At this time, there are four numbers, namely 1, 3,5,7,
(3) It can be understood that there is only 1-3, so in 1-3, 3 plus 1 equals 4, and then divided by 2 equals 2, which is the two numbers mentioned in (1), and the result is negative 2, and then it is assumed that there is only 1.
(4) At this time, we can understand that there is only one negative 2 in the case of two numbers, and there are two negative 2 in the case of four numbers, and then by analogy, 1-3+5-7+9 gets 2066 ...+2013-2015.