function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(0);

  stroke(255);

  line(200,200,mouseX,mouseY);

  var x = ave(200,mouseX);

  var y = ave(200,mouseY);

  fill(255,0,0);

  ellipse(x,y,30,30);

//   challenge: use the ave function again to make

//   a green dot half way between the center and the

//   red dot

  var x2 = ave(200,x)

  var y2 = ave(200,y)

  fill(0,255,0);

  ellipse(x2,y2,30,30);

}

function ave(x, y){

  return (x+y)/2; // replace with code to return average of the two arguments

}

​