Rotation by 45 degrees each time

1

I would like to be able to rotate a movieclip by 45 degrees each time i press left or right.

I have got this


on (keyPress "<Right>") {
 _rotation=+45
}
on (keyPress "<Left>") {
 _rotation=-45
}

However, this moves the movie clip to the point of 45 degrees or minus 45 degrees depending on the key pressed.

I would like to add +45 to the rotation when i press the right key and -45 when i press the left key.

What have i done wrong. Please can you help me?