You have `n` `tiles`, where each tile has one letter `tiles[i]` printed on it.

Return the number of possible non-empty sequences of letters you can make using the letters printed on those `tiles`.

Example 1:

    Input: tiles = "AAB"
    Output: 8
    Explanation: The possible sequences are "A", "B", "AA", "AB", "BA", "AAB", "ABA", "BAA".

Example 2:

    Input: tiles = "AAABBC"
    Output: 188

Example 3:

    Input: tiles = "V"
    Output: 1

Constraints:

- `1 <= tiles.length <= 7`
- `tiles` consists of uppercase English letters.