How to calculate multiple textboxes and handling NaN

var origValue = 0 //keeping pending value in original state

$scope.GetTotalAmount = function (data) {

var popDiscAmount = $scope.DiscAmount;

var popDiscAmount1 = $scope.DiscAmount1;

if (origValue == 0) {

origValue = $scope.ReceiptsViewModel.Amount;

}

$scope.ReceiptsViewModel.Amount = (origValue - popDiscAmount);

$scope.ReceiptsViewModel.Amount = ($scope.ReceiptsViewModel.Amount popDiscAmount1);

$scope.ReceiptsViewModel.Amount = ($scope.ReceiptsViewModel.Amount - $scope.AdjustAmount );

calculations are working properly,but when i changing discAmount to10 total value showing NaN, same for discAmount1,

but when i enter AdjustAmount 20, total value adjusting 1000-(10+10+20)=960 showing total value correctly, after closing popup

discAmount changed with 10, amount should subtract by 10 from amount , same for discAmount1 also ,