I'm saying kaddish for my mother-in-law. Occassionally, I walk into shul and the congregation is already in the middle of the Rabbi's Kaddish (say at Y'he sh'may raba"). Can I join in that kaddish at all, or have I lost out? If I can join, do I quickly say the parts I missed and try to catch up, or do I pick up from where the other mourners are at? What if I arrived on time, but I'm at a different point in the service?