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// aaaaa
function determine(x, y) {
  var a = Math.floor(x / 20);
  var b = Math.floor(y / 20);

  if(a % 2 == 0 && b % 2 == 0) {
    return true;
  } else if(a % 2 == 1 && b % 2 == 1) {
    return true;
  } else {
    return false; 
  }
}

function setup() {
 createCanvas(500, 500);
}

function draw() { 
  background(255);
  rectMode(CENTER);
 
  for(var y = 9; y < height; y += 20) {
    for(var x = 9; x < width; x += 20) {
      for(var d = 18; d > 0; d -= 4){
        if(determine(x,y)){
          rect(x, y, d, d);
        }
      }      
    }            
  }
}