With clever use of the general sibling selector:
#slide1:checked ~ #slides .inner { margin-left:0; }
#slide2:checked ~ #slides .inner { margin-left:-100%; }
#slide3:checked ~ #slides .inner { margin-left:-200%; }
#slide4:checked ~ #slides .inner { margin-left:-300%; }
#slide5:checked ~ #slides .inner { margin-left:-400%; }