Let g1 be greedy's first activity (ends earliest) and o1 be OPT's first activity.
Case 1: o1=g1. No change needed. Both agree on the first activity.
Case 2: o1=g1. Since g1 ends earliest, end(g1)≤end(o1). Replace o1 with g1 in OPT to create OPT'. Is OPT' valid? Yes! Since g1 ends no later than o1, it cannot conflict with OPT's second activity (which starts after P10 ends). OPT' has the same count as OPT.