import math


def get_board_size(game_board):
    """(str) -> int

    Precondition: board_size is a perfect square

    Return the side length of the tic tac toe board

    >>>get_board_size('x-ox')
    2
    >>>get_board_size('xoxoxxoxx')
    3
    """

    return int(math.sqrt(len(game_board)))