Width and height problem in XNA 4 Express Studio

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.NET4

XNA

I'm trying to create a XNA maze game, but at the very beginning, I got a problem. Here's my Update method:

if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed || Keyboard.GetState(PlayerIndex.One).IsKeyDown(Keys.Escape))
    this.Exit();
if (Keyboard.GetState(PlayerIndex.One).IsKeyDown(Keys.Left))
    ballPosition.X -= ballSpeed;
if (Keyboard.GetState(PlayerIndex.One).IsKeyDown(Keys.Right))
    ballPosition.X += ballSpeed;
if (Keyboard.GetState(PlayerIndex.One).IsKeyDown(Keys.Down))
    ballPosition.Y += ballSpeed;
if (Keyboard.GetState(PlayerIndex.One).IsKeyDown(Keys.Up))
    ballPosition.Y -= ballSpeed;
if (ballPosition.X >= width)
    ballPosition.X = width;
if (ballPosition.X <= 0)
    ballPosition.X = 0;
if (ballPosition.Y >= height)
    ballPosition.Y = height;
if (ballPosition.Y <= 0)
    ballPosition.Y = 0;
base.Update(gameTime);

But, when I run the game, ball won't go out at x=0 or y=0, but at other points it goes out of my game screen.
P. S. width and height are specified in Initialize method with ballPosition default values:

width = graphics.GraphicsDevice.Viewport.Width;
height = graphics.GraphicsDevice.Viewport.Height;
ballPosition.X = width / 2;
ballPosition.Y = height / 2;