Show that the circumference of the unit circle is 2 *integral from -1 to 1 of 1/sqrt(1-x^2)

hint: use the equation x^2+y^2=1 and the arc length formula

I simplified my answer down to the integral from -1 to 1 of 1/sqrt(1-x^2) but how do i get it to be two times the whole thing?