var x = 250;
var b = 10;
var y = 50;
var c = 10;

function setup() {
  createCanvas(400, 400);
}

function draw() {
  background(0);
  ellipse(x, y, 50, 50);
  if(x > width || x < 0) {
    b = -b;
  }
  if(y > height || y < 0) {

    c = -c;
  }
  y = y + c;
  x = x + b;
}