他说 suppose x is the total time that exactly one of $u_i$ and $v_i$ is playing with Tenzing，x <= $y_i$,那是说只能有一个小于等于$y_i$，另一个必须大于$y_i$吗？ 还是说至少有一个的时间必须要小于等于$y_i$呀