#! /usr/bin/python


def is_prime(n):
    """
    >>> is_prime(2)
    True
    >>> is_prime(3)
    True
    >>> is_prime(4)
    False
    >>> is_prime(20)
    False
    >>> is_prime(21)
    True
    >>> is_prime(43243432)
    False
    """
    if n < 3:
        return True
    elif n % 2 == 0:
        return False
    else:
        return True


def num_digits(n):
    """
    >>> num_digits(12345)
    5
    >>> num_digits(0)
    1
    >>> num_digits(-12345)
    5
    """
    count = 0
    if n == 0: return 1
    while n:
        count = count + 1
        n = abs(n) / 10
    return count


def num_even_digits(n):
    """
    >>> num_even_digits(123456)
    3
    >>> num_even_digits(2468)
    4
    >>> num_even_digits(1357)
    0
    >>> num_even_digits(2)
    1
    >>> num_even_digits(20)
    2
    """
    count = 0
    while n:
        i = n % 10
        num = (n - i) / 10
        if i % 2 == 0:
            count += 1
        n = num
    return count

if __name__ == '__main__':
    import doctest
    doctest.testmod()