Как сделать прыжок при нажатии вверх?

[Final Hope]

Добрый день, уважаемые форумцы. Я пытаюсь написать свою первую игру и никак не могу придумать, как заставить кружок прыгать. Помогите, пожалуйста. Буду весьма благодарен. Мой код показан ниже. Спасибо за внимание.

package
{
	import flash.display.MovieClip;
	import flash.display.Shape;
	import flash.events.Event;
	import flash.events.KeyboardEvent;
	import flash.geom.Point;
 
	public class Main extends MovieClip
	{
		private var _heroShape:Shape;
		private var _heroSpeed:Point = new Point(0, 0);
		private var _isLefting:Boolean = false;
		private var _isRighting:Boolean = false;
		private var _isUping:Boolean = false;
 
		private static const FLOOR:Number = 400;
		private static const SPEED_DELTA:Number = 1;
		private static const SPEED_DAMPING:Number = 0.9;
		private static const SPEED_MAX:Number = 5;
 
		public function Main()
		{
			drawHero();
			drawFloor();
			stage.addEventListener(Event.ENTER_FRAME, enterFrameHandler);
			stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDownHandler);
			stage.addEventListener(KeyboardEvent.KEY_UP, keyUpHandler);
		}
		//-------------------------------------------------------------
		//
		//  Methods
		//
		//-------------------------------------------------------------
		private function drawHero():void
		{
			_heroShape = new Shape();
			_heroShape.graphics.lineStyle(2);
			_heroShape.graphics.drawCircle(0, -30, 30);
			_heroShape.x = 275;
			_heroShape.y = FLOOR;
			this.addChild(_heroShape);
		}
		private function drawFloor():void
		{
			this.graphics.lineStyle(2);
			this.graphics.drawPath(new <int>[ 1, 2 ], new <Number>[ 0, FLOOR, 550, FLOOR ]);
		}
		//-------------------------------------------------------------
		//
		//  Event handlers
		//
		//-------------------------------------------------------------
		private function enterFrameHandler(event:Event):void
		{
			_heroShape.x += _heroSpeed.x;
			_heroShape.y += _heroSpeed.y;
			if (_isLefting == true && Math.abs(_heroSpeed.x) < SPEED_MAX)
			{
				_heroSpeed.x += -SPEED_DELTA;
				if (_heroSpeed.x < -SPEED_MAX)
				{
					_heroSpeed.x = -SPEED_MAX;
				}
			}
			else if (_isRighting == true)
			{
				_heroSpeed.x += SPEED_DELTA;
				if (_heroSpeed.x > SPEED_MAX)
				{
					_heroSpeed.x = SPEED_MAX;
				}
			}
			else
			{
				_heroSpeed.x *= SPEED_DAMPING;
			}
		}
		private function keyDownHandler(event:KeyboardEvent):void
		{
			switch (event.keyCode)
			{
				case 37:
				{
					_isLefting = true;
					break;
				}
 
				case 39:
				{
					_isRighting = true;
					break;
				}
 
				case 38:
				{
					_isUping = true;
					break;
				}
			}
		}
		private function keyUpHandler(event:KeyboardEvent):void
		{
			switch (event.keyCode)
			{
				case 37:
				{
					_isLefting = false;
					break;
				}
 
				case 39:
				{
					_isRighting = false;
					break;
				}
 
				case 38:
				{
					_isUping = false;
					break;
				}
			}
		}
	}
}