rotation between two images automatically with SIft ??

I have two images, the first is a reference and the second is an image rotated. i have used the the fct : - VL_SIFT(I) computes the SIFT descriptors in both images so the geometry information is provided

[f1,d1] = vl_sift(im1g) ; [f2,d2] = vl_sift(im2g) ; - match the interest point by nearest neighbor algorithm.

i would like to know after matching how can i recognize the angel of rotation between that tow images automatically

thanks