function checkSize() {
  var nav = document.getElementById("nav-container");
  var content = document.getElementById("content");
  if (nav.offsetHeight && content.offsetHeight && nav.offsetHeight > content.offsetHeight)
    content.style.height = nav.offsetHeight -(-50) + "px";
  else if(nav.style.pixelHeight && content.style.pixelHeight && nav.style.pixelHeight > content.style.pixelHeight)
    content.style.height = nav.style.pixelHeight -(-50) + "px";
}

window.onload = checkSize;