function boxCheck() {
  if (document.body.clientWidth < 830) {
    document.getElementById("box").style.position = "relative";
    document.getElementById("box").style.top = "0px";
    document.getElementById("box").style.left = "0px";
  } else {
    document.getElementById("box").style.position = "absolute";
    document.getElementById("box").style.top = "300px";
    document.getElementById("box").style.left = "780px";
  }
}
window.onresize=boxCheck;