Does anyone know how to write a union for the type hinting?

I am doing the following, but it is not being recognised by PyCharm:

```python
def add(a, b)
    # type: (Union[int,float,bool], Union[int,float,bool]) -> Union([int,float,bool])
    return a + b

```

What is the correct way of specifying a type hint for a union?

I am using python 2.7 for this.