23. 32 students apply to participate in an international exchange activity, only 2 can participate, the selection method is that the 32 students stand in a line, and count from 1 from front to back, those who report odd numbers are eliminated and exit the queue, then count from 1 from front to back again. Xiao Ming very much wants to participate in this activity, to ensure that he is selected, the number he should report when lining up for the first time is or $\qquad$