int x=0;
int y=100;

void setup() {
  size(400,400);
  background(200,0,0);
  ellipseMode(CENTER_DIAMETER);
}

void loop() {
  background(200,0,0);
  fill(255,255,255);
  noStroke();
  ellipse(x,y, 20,20);
  x=x+10;
  y=y+3;
  if(x>width) {
    x=0;
    y=y-10;
  }
  if(y>height) y=0;
  else if(y<0) y=height;
}