var sel = document.getElementById("sel");
var inp = document.getElementsByTagName("input")[0];

	inp.value = sel.options[sel.selectedIndex].text;
	sel.addEventListener("change", function() {

	var opts = sel.options;
	for (var i = 0; i < opts.length; i++) {
		if (opts[i].selected == true) {
			inp.value = opts[i].text;
		}
	}
	});

  html______________________________________
  <select name="my" id="sel">
		<option value="one">1</option>
		<option value="two">2</option>
		<option value="three">3</option>
	</select>
	<input type="text" />