void draw() {
background(50);  
fill(255,255,255);
rect(0,0,300,rectY_2);
fill(255,0,0);
rect(0,rectY_2,300,rectY);
}

void keyPressed() {
if (keyCode == UP) {
rectY-=50;
rectY_2+=50;
}
else if (keyCode == DOWN) {
rectY+=50;
rectY_2-=50;
}

}