def next_bet(current, strategy):
    """Returns the next bet based on the current bet and betting strategy.
    Standard increases by AUD denominations until reaching 100. Non standard
    betting strategy doubles each time

    Parameters:
    current (int): The current bet to be increased for the next roll
    strategy (string): 'standard' or 'exponential' which formula to use
    to determine next bet

    Returns
    int: The value of the next bet.
    """
    if strategy == 'standard':
        if current is None:
            return 5
        elif current == 5:
            return 10
        elif current == 10:
            return 20
        elif current == 20:
            return 50
        else:
            return 100
    else:
        if current is None:
            return 5
        else:
            return 2*current