from random import choice

class Student:
    
    assess_values = ['B-', 'B+', 'A-' 'A+']
    
    def __init__(self, first_name, last_name):
        self.first_name = first_name
        self.last_name = last_name
        self.grades = []

    @property
    def full_name(self):
        return f'{self.first_name} {self.last_name}'

    @property
    def email(self):
        return f'{self.first_name}{self.last_name[0]}@codewars.com'

    def add_grade(self, grade):
        self.grades.append(grade)
        
    @property
    def grade_average(self):
        if not self.grades:
            return 0
        return sum(self.grades) / len(self.grades)
    
    def assess(self):
        if not self.grades:
            return 'Ungraded'
        
        return self.assess_values[self.grade_average // 25]
        
            
    def add_random_grade(self):
        random_grade = choice(range(1, 11))
        self.grades.append(random_grade)

unsigned long long div2(unsigned long long a) {
  return a >> 1;
}

import math

def shannon_entropy(s):
    symbols = list(s)
    unique_symbols = set(s)
    M = float(len(s))
    return -(sum([a * (math.log(a, 2)) for a in [symbols.count(x)/M for x in unique_symbols]]))

is_even=lambda n:int(not(n&True))