def vaf_to_ar(vaf):
    """
    Convert VAF to AR.

    VAF (variant allele frequency) = V-AF
    AR (allele ratio) = V-AF / WT-AF
    V-AF + WT-AF = 100 (%)

    Note:
        if VAF == 100:
            AR = -1
            (instead of 100 / 0)

    Args:
        vaf (dc.Decimal): VAF to convert.

    Returns:
        AR (dc.Decimal)
    """
    if vaf == 100:
        return -1
    return vaf/(100 - vaf)