Letter Tile Possibilities

Statement

You are given a string, tiles, consisting of uppercase English letters. You can arrange the tiles into sequences of any length (from 1 to the length of tiles), and each sequence must include at most one tile, tiles[i], from tiles.

Your task is to return the number of possible non-empty unique sequences you can make using the letters represented on tiles[i].

Constraints:

• $1 \leq$ tiles.length $\leq 7$

• The tiles string consists of uppercase English letters.

Examples