s-0
s-1
s-2
s-3
su-1
x-2
sum-1
http://prezi.com/hps-jm4tv99k/present/?auth_key=l3pp6kw&follow=zaeus40enspx
sum1.pptx
sum197.ppt
http://prezi.com/hps-jm4tv99k/present/?auth_key=yennzgt&follow=zaeus40enspx&kw=present-hps-jm4tv99k&rc=ref-3125240