Any two or more incorporated cities wishing to take advantage of sections 71.370 to 71.390 may, by ordinance duly enacted in each of such cities, agree upon the terms upon which such fire protection shall be furnished, and such agreement may, where two or more such cities have fire departments, include an interchange of the service of such fire departments upon such terms as are agreed upon; or such agreement may provide for the payment of a stated sum per month or per year, or a stated sum per fire, or any other method of compensation for such fire protection that is agreed upon by the two or more incorporated cities entering into such contract; provided, that any contract for a longer period than five years shall have no binding force until ratified by a majority of the voters voting on the question in each of the cities entering into such contract.