The expression (-1)^n means the number -1 multiplies itself n times.
So for example if n = 2, we have that:
For n = 3, we have:
For n = 4:
We can see that the result alternates from -1 and 1, and when n is odd, the result is -1, and when n is even, the result is 1.
So for any even number n, the result will be 1.