function openTutorial() {
 x = (screen.width - 760) / 2;
 y = (screen.height - 500) / 2;
 var w = window.open(
 "/tutorial_en.html",
 "NinjaTrickGame",
 "screenX=" + x + ",screenY=" + y + ",width=760,height=500,toolbar=no,location=no,statusbar=no,menubar=no,scrollbars=no,resizable=no,titlebar=no"
 );
 w.focus();
}