How to stop an if statement

Im really bad at this and Im trying to make a "simulation" of people entering and exiting a room. When the mouseX > width/2 then a "new person comes in" and when mouseX > width/2 then the program should remove a person.

It "works", but it never stops adding/subtracting "people". I don't know how to make it add or subtract only one time for every-time the mouse crosses the canvas.

here's the code:

String comesIn = "In";
String goesOut = "Out";

int n=0;

void setup() 
  {
  size(640, 360);
  }

void draw()
{
background(255);
line(320, 0, 320,360 );

    if(mouseX > width/2)
    {
      println("in");
      text(comesIn, (width/2)+(width/4), height/2);
      fill(50);
      n++;
      println("people = "+n);
    }
    if(mouseX < width/2)
    {
      println("out");
      text(goesOut, width/4, height/2);
      fill(50);
      n--;
      println("people = "+n);
    } 
}

THANK YOUUU