Let a be a real number...?

Let 'a' be a real number such that a 1 < a < 2. a(sub n) is the sequence defined by:

a1 = a,

an+1 = |an| -1

n= 1,2,3......

And put Sn= a1+a2....+an

Find. a4, a5 a6 a7

Find S2, S4, S6

I'm not following the problem at all O.o

the answers are a4 = 1-a

a5 = a -2

a6 = 1 -a

a7 = a-2

Maybe if i just get an example to solve a4 I might be able to figure out the rest...