def moretrees(a_dict):
    """assumes a_dict is a dictionary of key strings and value ints,
    representing country names and number of tree per square kilometer
    returns a list of strings, representing the name of countries
    with more than 20.000 trees per square kilometer"""
    green_contries_list = []
    for key, value in a_dict.items():
        if value >= 20000:
            green_contries_list.append(key)
    return green_contries_list