birt,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<table id="v_table">
<tbody>
<tr>
<td>1</td><td><input type="text" name="nep_1" id="nep_1"></td>
</tr>
</tbody>
<tfoot>
<tr><td colspan="2"><center><button onclick="add_value_f()">+</button></center></td></tr>
<tr><td colspan="2"><center><input type="text" name="sum" id="sum"></td></tr>
</tfoot>
</table>
<script>
var d = document;
var last_id = 1;
function add_value_f() {
var tbody = d.getElementById("v_table").getElementsByTagName("tbody")[0];
var row = d.createElement("tr");
tbody.appendChild(row);
var td1 = d.createElement("td");
var td2 = d.createElement("td");
row.appendChild(td1);
row.appendChild(td2);
last_id = last_id + 1;
td1.innerHTML = +last_id + "";
td2.innerHTML = '<input type="text" name="nep_' + last_id + '" id="nep_' + last_id + '">'
}
var table = d.getElementById("v_table"),
sum = d.getElementById("sum");
table.addEventListener("input", function() {
sum.value = [].reduce.call(table.querySelectorAll('[name^="nep_"]'), function(summ, item) {
return summ += +item.value || 0
}, 0)
});
</script>
</body>
</html>