def priority_to_severity(priority):
    """
    coverts NetWitness priority to Demisto severity grade

    input:
        - 'Low'
        - 'Medium'
        - 'High'
        - 'Critical'
    output:
        - 0 Unknown
        - 1 Low
        - 2 Medium
        - 3 High
        - 4 Critical
    """

    priority_grade_map = {
        'Low': 1,
        'Medium': 2,
        'High': 3,
        'Critical': 4
    }

    grade = priority_grade_map.get(priority, 0)
    return grade