Pope John XXIII did not live to see the Vatican Council to completion. He died of stomach cancer on 3 June 1963, four and a half years after his election and two months after the completion of his final and famed encyclical, Pacem in terris. He was buried in the Vatican grottoes beneath Saint Peter's Basilica on 6 June 1963 and his cause for canonization was opened on 18 November 1965 by his successor, Pope Paul VI, who declared him a Servant of God. In addition to being named Venerable on 20 December 1999, he was beatified on 3 September 2000 by Pope John Paul II alongside Pope Pius IX and three others. Following his beatification, his body was moved on 3 June 2001 from its original place to the altar of Saint Jerome where it could be seen by the faithful. On 5 July 2013, Pope Francis – bypassing the traditionally required second miracle – declared John XXIII a saint, after unanimous agreement by a consistory, or meeting, of the College of Cardinals, based on the fact that he was considered to have lived a virtuous, model lifestyle, and because of the good for the Church which had come from his having opened the Second Vatican Council. He was canonised alongside Pope Saint John Paul II on 27 April 2014. John XXIII today is affectionately known as the "Good Pope" and in Italian, "il Papa buono".
Try to answer this question if possible (otherwise reply "unanswerable"): What illness did Pope Francis die of?
unanswerable