How about:
if (numKeys > 1){
t1 = key(1).time;
t2 = key(2).time;
v1 = thisComp.layer("Slider").effect("value")("Slider");
v2 = 0;
t = time<=t2 ? time : (Math.floor((time-t1)/(t2-t1)) % 2 === 0 ? t1+((time-t1) % (t2-t1)) : t2-((time-t1) % (t2-t1)));
linear(t,t1,t2,v1,v2);
}else
value