If you don't choose the monitor, that is, choose m out of n people and arrange them well, then Anm.

When choosing a monitor, that is, if m- 1 person is selected from n people and arranged side by side, then an-1m; If the display has m options, then man-1m.

Then anm+man-1m = an+1m.

Here, the monitor is called a special element.