Passage: Ali was born inside the sacred sanctuary of the Kaaba in Mecca, the holiest place in Islam, to Abu Talib and Fatimah bint Asad. He was the first male who accepted Islam, and, according to some authors, the first Muslim. Ali protected Muhammad from an early age and took part in almost all the battles fought by the nascent Muslim community. After migrating to Medina, he married Muhammad's daughter Fatimah. He was appointed caliph by Muhammad's companions in 656, after Caliph Uthman ibn Affan was assassinated. Ali's reign saw civil wars and in 661, he was attacked and assassinated by a Kharijite while praying in the Great Mosque of Kufa, being martyred two days later.Ali is important to both Shias and Sunnis, politically and spiritually. The numerous biographical sources about Ali are often biased according to sectarian lines, but they agree that he was a pious Muslim, devoted to the cause of Islam and a just ruler in accordance with the Qur'an and the Sunnah. While Sunnis consider Ali the fourth and final of the Rashidun (rightly guided) caliphs, Shia Muslims regard Ali as the first Imam after Muhammad due to their interpretation of the events at Ghadir Khumm. Shia Muslims also believe that Ali and the other Shia Imams (all of whom are from the Ahl al-Bayt, Muhammad's household) are the rightful successors to Muhammad. Ali has also received recognition from a variety of non-Muslim organizations, such as the United Nations and the World Organization for Human Rights, for his governance and social justice.

Question: How many caliphs do Sunnis believe exist?
four