function setup() {

  createCanvas(400, 439);

  topp=width*0.25

  bottom=width*0.75

  left=height*0.25

  right=height*0.75

}

​

function draw() {

  background(220);

  line(topp,left,topp,right)

  line(topp,right,bottom,right)

  line(bottom,right,bottom,left)

  line(bottom,left,topp,left)

}