According to the meaning of the question, we know that the "total" can only be a few numbers between -7 and +7 (that is, -7, -6, -5, -4, -3, -2,-1, 0, 1, 2, 3, 4, 5, 6, 7.

Then "the change of sum" means that the superposition of the above values is equal to 2 or -2!

For example, "-5"+"7" = 2,

"4"+"-6"=-2、

I hope to have a reference understanding for you, thank you!