**Left factorials**, $ !n $, may refer to either *subfactorials* or to *factorial sums*. The same notation can be confusingly seen used for the two different definitions. Sometimes, *subfactorials* (also known as *derangements*) may use any of the notations:
<ul>
<li>$!n`$</li>
<li>$!n$</li>
<li>$n¡$</li>
</ul>
(It may not be visually obvious, but the last example uses an upside-down exclamation mark.) This task will be using this formula for **left factorial**:
$ !n = \\sum\_{k=0}^{n-1} k! $
where $!0 = 0$
# --instructions--
Write a function to calculate the left factorial of a given number.
# --hints--
`leftFactorial` should be a function.
```js
assert(typeof leftFactorial == 'function');
```
`leftFactorial(0)` should return a number.
```js
assert(typeof leftFactorial(0) == 'number');
```
`leftFactorial(0)` should return `0`.
```js
assert.equal(leftFactorial(0), 0);
```
`leftFactorial(1)` should return `1`.
```js
assert.equal(leftFactorial(1), 1);
```
`leftFactorial(2)` should return `2`.
```js
assert.equal(leftFactorial(2), 2);
```
`leftFactorial(3)` should return `4`.
```js
assert.equal(leftFactorial(3), 4);
```
`leftFactorial(10)` should return `409114`.
```js
assert.equal(leftFactorial(10), 409114);
```
`leftFactorial(17)` should return `22324392524314`.
```js
assert.equal(leftFactorial(17), 22324392524314);
```
`leftFactorial(19)` should return `6780385526348314`.
