In the aftermath of the 1904 revolution, the Paraguayan liberal movement entered a period of instability and factional fighting. Between 1904 and 1922, 15 Paraguayan presidents assumed office. The coups of 1908 and 1910 cemented the division of Paraguayan liberals into the "radicales" and "civicos" factions. A further split of the radicales faction into two sub-factions led by Manuel Gondra and Eduardo Schaerer respectively, gradually escalated into a full-scale civil war. In 1920, Gondra came out victorious in the presidential election. Schaerer immediately attempted to undermine Gondra by forcing the resignation of Interior Minister José Guggiari, Gondra's most vocal supporter. After Gondra's refusal to dismiss Guggiari, Gondra himself resigned, due to the emergence of a split in Paraguay's armed forces. After Vice President Félix Paiva's refusal to assume office, the congress placed senator Eusebio Ayala, a moderate Gondra supporter into interim presidency. Another crisis ensued when Adolfo Chirife, a supporter of Schaerer, became a presidential candidate. Amassing the support of the Colorado party along with a large percentage of supporters in the Liberal Party, Chirife gathered enough support to ensure a victory in the upcoming elections. When Ayla reacted by blocking implementation of the elections, Schaerer's supporters attempted to restore the electoral process by force.
Question: How many years did 15 Paraguayan assume office between 1904 and 1922?
Answer: 18