Hi I have written the following program math.ex

```
defmodule Math do
def mul(_, 0) do
0
end
def mul(0, _) do
0
end
def mul(a, b) when b < 0 do
sum = a + mul(a, b+1)
sum*(-1)
end
def mul(a, b) do
sum = a + mul(a, b-1)
sum
end
end
```

Now, when it try to the do `Math.mul(4,-10)`

the expected answer is the -40 but it gives me 0. Also if i change the line 7 to just return `sum`

instead of `sum*(-1)`

I get `40`

.