AS2 Mouse speed detecting script

var check:Boolean = true;
var speed:Number;

wait = setInterval(go,200);//will check aver 200 milliseconds

function go(){
if(check){
X1= _xmouse;
check = false;
}else{
X2=_xmouse;
speed = (X2-X1)
check = true;
trace(speed);
};
}