Calculate the Sum of Values using ng-repeat in Angular js - angularjs

Question

I am newbie to Angular js.I am just trying to calculate the sum of Values of each row using angular js.Here, I am dynamically adding and removing rows.
For Eg: Let's say first row value is 30, Now I added one row with value 30, then total should be show 60 like this.
1. 30
2. 30
3. 50
Total 120
HTML Code:
<table id="t1" style="border:none;">
<tr><th>Start</th><th>Stop</th><th>Downtime(In Min)</th><th>Reason</th></tr>
<tr ng-repeat="item in invoice">
<td><input type="text" required ng-model="$invoice.start" name="r1[]"></td>
<td><input type="text" required ng-model="$invoice.stop" name="r2[]" ng-blur="diff($invoice)"></td>
<td><input type="text" name="r3[]" ng-model="$invoice.diff"/></td>
<td style="border:none;"><a href ng-click="remove(item)">X</a></td>
</tr>
<tr style="border:none;">
<td style="border:none;"><a href ng-click="add()">+</a></td>
</tr>
</table>
<span class="labelCode">Total Downtime</span><input required type="text" name="Tot_D" ng-value="d1()"/></span></br>
Angular JS:
$scope.d1=function(){
var total = 0;
for(var i = 0; i < $scope.invoice.length; i++){
var product = $scope.invoice[i];
total += parseInt(product.diff);
}
return total;
}
//Calculate the Difference between two times
$scope.diff = function(item) {
item.diff = computeDiff(item.start,item.stop);
}
function computeDiff(start, stop) {
if (start && stop) {
var s_hr = start.split(":")[0];
var s_min = start.split(":")[1];
var e_hr = stop.split(":")[0];
var e_min = stop.split(":")[1];
return Math.abs((parseInt(e_hr) - parseInt(s_hr)) * 60) + Math.abs(parseInt(e_min) - parseInt(s_min))
}
}
Now, it just showing NaN. I don't know what is wrong.
Please help out.Thanks in advance.