sequence 40, -20, 10. -5... is geometric

find Un and hence find the 10th term

what i got was

u1 = 40

and r = 1/2

so --> un=u1xr^n-1

un=40x(-1/2)^10-1

un=-20^9

un= -0.00000000000512

but the correct answer is -5/256

--> could you tell me where i got wrong?