CSS3 siblings and first-child

Hello Experts,

For testing purpose: I am trying to apply a style on the "a" tags that will ONLY apply on the main links and NOT on the sub links (in the inner UL).  I was trying several ways but or it apply to all the "a" tags or it does not apply at all ... e..g....
I would be happy to know what am i doing wrong.

nav ul:first-child li a {
	color:red;
}

nav ul:first-of-type li a {
	color:red;
}

nav + u li a {
	color:red;
}

<nav>

  <ul>
    <li><a href="#">link 1</a></li>
    <li><a href="#">link 2</a></li>
    <li><a href="#">link 3</a>

      <ul>
        <li><a href="#">sub link 1</a></li>
        <li><a href="#">sub link 2</a></li>
      </ul>

    </li>
  </ul>

</nav>