int size = 50;
void setup(){
size (600, 600);
}
void draw(){
for (int h = 0; h < width; h += size) {
for (int v = 0; v < height; v += size) {
if ((h+v) % 20 == 0) {
fill(255);
}
else {
fill(0);
}
rect (h, v, size, size);
}
}
}