## Optional Sum
### Instructions
Write a function named `optionalSum` that accepts two `int` arguments, and an optional `int` argument. Return the sum of all the arguments.
### Optional parameters
In Dart, you can make function arguments optional, meaning that a function can work even if the optional argument is omitted. If the optional parameter is omitted, it is considered to be `null`.
A function with optional parameters:
```dart
void someFunction(int first, int second, [int? third]) {
if (third != null) {
print('${first}, ${second}, ${third}');
} else {
print('${first}, ${second}');
}
void main() {
someFunction(1, 2);
someFunction(1, 2, 3);
```
> Optional parameters must come after the required parameters.
> You cannot use both optional and named parameters.
