Setting Up a Simple GUI

int screen=0;

void setup()
{
  size(400,400);
  background(color(255));
  smooth();
}

void draw()
{  
  background(255);
  if (screen == 0)
  {
    fill(color(255));
    rect(50,50,80,25);
    rect(250, 50, 80, 25);
    fill(color(0));
    text("Play Game", 60, 70);
    text("Instructions", 260, 70);
  }
  else if (screen == -1)
  {
    background(200);
    text("Instructions:", 50, 100);
    text("1) Score as many points as you can", 50, 150);
    text("2) Don't die",50, 180);
    text("Click the mouse to return to menu", 50, 250);
  }
  else
  {
    text("Now we play the game", 200, 200);
  }
}

void mouseReleased()
{
  if (screen == -1)
    screen = 0;
  else if (mouseX >= 50 && mouseX <= 130 && mouseY >= 50 && mouseY <= 75)
    screen = 1;
  else if (screen != 1 && mouseX >= 250 && mouseX <= 330 && mouseY >= 50 && mouseY <= 75)
    screen = -1;
}