int a = 0;
int b = 0;

void setup() {
  size(200, 200);
}

void draw() {
  fill(255);
  rect(0, 0, width, height);
  fill(0);
  line(width/2, 0, width/2, height * 3 / 4);
  line(0, height * 3 / 4, width, height * 3 / 4);
  text(a, width / 4, height / 4);
  text(b, width * 3 / 4, height / 4);
  text("Sum: " + (a + b), width / 4, height * 7 / 8);
}

void mousePressed() {
  if (mouseX < width/2) {
    a++;
  } else {
    b++;
  }
}