--(-k)------------(-3)-------(-2)------(-1)-----0----(+1)---(+2)---(+3)---------------(+k)--
You will notice that all values on the left of "0" are negative and positive on the right .
Moreover
+1 is symmetric to -1 about "0"
+2 is symmetric to -2 about "0"
+3 is symmetric to -3 about "0"
and +k is symmetric to -k about "0"
So the opposite of -k is +k and it's symmetric of -k vs."0"
