def add_in_dict(dict1, dict2):
    """Merge dictionary keys and add the content from dict1 and dict2

    :param dict1: first dictionary
    :param dict2: second dictionary
    :returns: merged and added dictionary

    >>> add_in_dict({'a':1, 'b':2}, {'c':3, 'd': 4}) == {'d': 4, 'a': 1, 'c': 3, 'b': 2}
    True
    >>> add_in_dict({'a':1, 'b':2}, {'a':3, 'b': 4}) == {'a': 4, 'b': 6}
    True
    >>> add_in_dict({'a':1, 'b':2}, {'a':3, 'c': 4}) == {'a': 4, 'b': 2, 'c': 4}
    True

    """
    new_dict = {}
    new_dict.update(dict1)
    for (k,v) in dict2.items():
        if k in new_dict.keys():
            new_dict[k] += v
        else:
            new_dict[k] = v

    return new_dict