Part II Regional Instruments the basis of a written request addressed to the Secretary General of the Organization of African Unity. The withdrawal shall take effect six months after the date of receipt of the written request by the Secretary General of the Organization of African Unity. Article 20 1. This Convention shall enter into force thirty days after the deposit of the fifteenth instrument of ratification with the Secretary General of the Organization of African Unity. 2. For each of the States that shall ratify or accede to this Convention shall enter into force thirty days after the date of the deposit by that State Party of its instrument of ratification or accession. Article 21 1. Special protocols or agreements may, if necessary, supplement the provisions of this Convention. 2. This Convention may be amended if a State Party makes a written request to that effect to the Secretary General of the Organization of African Unity. The Assembly of Heads of State and Government may only consider the proposed amendment after all the States Parties have been duly informed of it at least three months in advance. 3. The amendment shall be approved by a simple majority of the States Parties. It shall come into force for each State which has accepted it in accordance with its constitutional procedures three months after the Secretary General has received notice of the acceptance. Article 22 1. Nothing in this Convention shall be interpreted as derogating from the general principles of international law, in particular the principles of international humanitarian law, as well as the African Charter on Human and Peoples' Rights. 2. Any dispute that may arise between the States Parties regarding the interpretation or application of this Convention shall be amicably settled by direct agreement between them. Failing such 218