Subtract the substring

On the board, a natural number $n$ is written. Akim Sergeyevich and Masha from A' take turns making moves.

During each turn, a player selects a natural number $m$ which is a proper contiguous substring of the number currently displayed on the board, and subtracts $m$ from it.

For example, if the number on the board is $2309$, a player can choose $m$ to be 2, 3, 9, 23, 30, 230 or $309$. Consequently, after this move, the number on the board will become one of 2000, 2079, 2279, 2286, 2300, 2306 or $2307$.

The player who cannot make a move loses.

Akim Sergeyevich makes the first move. Help him defeat Masha! Find the minimum number $m$ he should subtract on his first move to ensure a win (assuming Masha plays optimally).