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

function draw() {
  let millisecond = millis();
  let num_of_square = 10 - Math.floor((millisecond / 500) % 10);
  background(220);

  var j = 0;
  for (var i = 10; i > 0; i--) {
    console.log(j);
    var top_left_x = 0 + 20 * j;
    var top_left_y = 0 + 20 * j;
    var square_length = 400 - 40 * j;

    if (j + 1 >= num_of_square) {
      if (j % 2 == 0) {
        const c = color(0, 0, 0);
        fill(c);
      } else {
        const c = color(255, 255, 255);
        fill(c);
      }
      // square(20 * j, 20 * j, 400 - 20 * 2 * j);
      square(top_left_x, top_left_y, square_length);
    }
    j += 1;
  }
}