A positive integer n is written on a board, and two players take turns making moves.
In each move, the player selects a positive integer m that is a proper substring of the number currently written on the board. The number on the board is then decreased by m. A proper substring is a substring that doesn't equal the whole string.
For example, if a board contains the number 2309, a player can select m = 2, 3, 9, 23, 30, 230 or 309. Thus the possible moves from 2309 are 2000, 2079, 2279, 2286, 2300, 2306 and 2307.
A player who can't make a valid move loses the game.
Return the number m that the first player should select in his first move in order to win the game. If there are several such numbers, return the smallest among them. If the first player cannot win the game, return -1.