def IR(spot:list, m=1):
    """
    IR(): A function to calculate Single Effective Interest Rate from an array of spot rates.

    :param spot: An array/List of Spot rates 
    :type spot: list
    :param m: Frequency of Interest Calculation (eg: 2 for Semi-annually), defaults to 1.
    :type m: float
    :return: float, None for -ve values of m.
    :rtype: float
    """
    if(m<=0 or len(spot)==0):
        return None
    return spot[-1]