War crimes were committed by both sides during the Macedonian struggle. According to a 1900 British report compiled by Alfred Biliotti, who is considered to have heavily relied on Greek intelligence agents, starting from 1897, the members of the Exarchist committees had embarked upon a systematic and extensive campaign of executions of the leading members of the Greek side. Moreover, Bulgarian Komitadjis, pursued a campaign of extermination of Greek and Serbian teachers and clergy. On the other hand, there were attacks by Greek Andartes on many Macedonian Bulgarian villages, with the aim of forcing their inhabitants to switch their allegiance from the Exarchate back to the Patriarchate and accept Greek priest and teachers, but they also carried out massacres against the civilian population, especially in the central parts of Macedonia in 1905 and in 1906. One of the notable cases was the massacre at the village Zagorichani , which was an aggressive pro-Bulgarian Exarchist stronghold near Kastoria on 25 March 1905, where between 60 and 78 villagers were killed by Greek bands. According to British reports on political crimes , during the period from 1897 to 1912 over 4000 political murders were committed , excluding those killed during the Ilinden uprising and the members of the Bulgarian and Greek bands. Of those who were killed, 53% were Bulgarians, 33.5% were Greeks, Serbs and Aromanians together 3.5% and 10% were of an unknown nationality. These conflicts ended after the revolution of Young Turks in July 1908, as they promised to respect all ethnicities and religions, and to provide a constitution.
Question: How many years was the time span in which over 4000 political murders were committed according to British reports?
Answer: 15