def ordinalStr(n) :
    """
    Given the positive integer n, return the corresponding ordinal number string
    """
    assert(n>0)

    def getSuffix(n) :
        def getOrdKey(n) :
            if n < 14 : return n
            else      : return (n % 10)

        i = getOrdKey(n)
        if   i == 1: return 'st'
        elif i == 2: return 'nd'
        elif i == 3: return 'rd'
        else :       return 'th'

    return str(n) + getSuffix(n)