Find the curvature "k" of r(t) = <-t+sint , 1+cost>

I'm using the formula: (magnitude of cross product of a and v) / ((magnitude of velocity)^3)

I got:

v(t) = <-1+cost, -sint>

a(t) = <-sint, -cost>

mag. of cross prod. = 1+cost

mag. of velocity = sqrt(2-2cost)

k = (1+cost)/((2-2cost)^(3/2))

This answer is not correct. What am I doing wrong?