# Unique Ab Strings

You are given a string `s`

of `"a"`

and `"b"`

s. `"a"`

s can stay `"a"`

or turn into `"b"`

, but `"b"`

s can’t change.

Return the number of unique strings that can be made.

**Constraints**

`1 ≤ n ≤ 10,000`

where`n`

is the length of`s`

https://binarysearch.com/problems/Unique-Ab-Strings

## Examples

### Example 1

**Input**

- s =
`abba`

**Output**

- answer =
`4`

**Explanation**

We can make these strings:

`"abba"`

`"abbb"`

`"bbba"`

`"bbbb"`

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