def find_uninflected_stem(stem, form):
    """
    Finds all the shared caracters from left to right.
    find_uninflected_stem('rAmaH', 'rAmo') => -1+aH

    :param stem: form to reach by applying the diff
    :param form: given form
    :return: a diff: '-<number of chars to delete>+<characters to add>'
    """
    i = 0
    while i <= len(stem) - 1 and i <= len(form) - 1 and stem[i] == form[i]:
        i += 1
    stem_ending = stem[i:]
    form_ending = form[i:]
    if stem_ending == '' and form_ending == '':
        operation = ''
    else:
        form_ending_len = len(form_ending)
        operation = '-{}+{}'.format(form_ending_len, stem_ending)
    return operation