def count_matches(s1, s2):
    '''Return the number of chars in s1 that appear in s2.'''

    common_chars = 0
    for char in s1:
        if char in s2:
            common_chars += 1

    return common_chars


def ask_and_match_repeat():
    ''' Prompt the user to enter two strings and find out
        how many characters they have in common.  Repeat
        this task until the count is greater than 0, and return
        the count as an int'''

    common_count = 0
    while common_count == 0:
        word1 = raw_input("Enter a string: ")
        word2 = raw_input("Enter a string: ")
        common_count = count_matches(word1,word2)

if __name__ == '__main__':
    ask_and_match_repeat()