int s;


void setup() {
  size(400, 500);
  framerate(3);
  s = 30;
  
}

void draw() {
  
line(0,0,width,s);
line(width,0,width - s,height);
line(0,height,s,0);
line(width,height,0,height - s);

  if(s <= width){
    s = s + 10;
  }else{
    noLoop();
  }  
  
  }