Given two binary strings, return their sum (also a binary string).

Example

```
a = 11
b = 1
a + b:
011
+ 001
------
100
```

We can use a simple approach that adds one bit at a time. The following code use the following variables.

`ia`

: the current bit of `a`

`ib`

: the current bit of `b`

`carry`

: the carry