Hints:
Note that you will need to use the result
If you have studied complex variables, you obtain this by shifting the
contour. If not, you can take 
and by noting that
the result may be put in the form of an integral of an exact
derivative, prove 
and hence 
. Finally, you show that 
using the
standard trick of changing to polar coordinates to show
.
Also, you will face manipulations of complex numbers such as
and
.