int i;
int direction;

void setup(){
  size(300,300);//set the size of the stage
  direction = 1;

}
void loop(){
  background(0,255,255);
  ellipse(0,i,300,i);
  i += direction;

  if(i > 300){
    direction = -1;
  }
  if(i < 0){
    direction = 1;
  }

}