submission by jtoy
function setup(){
  var myCanvas = createCanvas(windowWidth,windowHeight); //you must keep this line unmodified
  rect(width/2,height/2,50,50)

  x = 0;
  y = 0;
}

function draw() {
  //start writing your code here

  x = x + random(5)
  y = y + random(5)

  fill(0,255,0)
  rect(x,y,20,20);
  fill(255,0,0)
  rect(width-x,height-y,20,20);
  fill(255,0,255)
  rect(x,height-y,20,20);
  fill(0,255,255)
  rect(width-x,y,20,20);
}