a(0)==1
a(1) == -1
a(2)== - 1 + 2*1 ==1
a(3)==1 + 2*- 1 ==-1
a(4)== - 1 + 2*1 == 1
a(5)==1 + 2*- 1 == - 1........and so on
Then the sequence is: 1, - 1, 1, - 1, 1, - 1.........etc.
All EVEN terms end in 1 and all ODD terms end in - 1
So: a(100)== - 1 + 2*1 ==1