Task: Question Answering
Input (English): Passage: The war is known by several names. "Polish-Soviet War" is the most common but other names include "Russo-Polish War  of 1919-1921"  and "Polish-Bolshevik War". This second term ) is most common in Polish sources. In some Polish sources it is also referred as the "War of 1920" . There is disagreement over the dates of the war. The Encyclopædia Britannica begins its article with the date range 1919-1920 but then states, "Although there had been hostilities between the two countries during 1919, the conflict began when the Polish head of state Józef Pilsudski formed an alliance with the Ukrainian nationalist leader Symon Petlyura  and their combined forces began to overrun Ukraine, occupying Kiev on 7 May." The Polish Internetowa encyklopedia PWN, as well as Western historians such as Norman Davies, consider 1919 the starting year of the war. The ending date is given as either 1920 or 1921; this confusion stems from the fact that while the ceasefire was put in force in the autumn of 1920, the official treaty ending the war was signed months later, in March 1921. While the events of 1919 can be described as a border conflict, and only in early 1920 did both sides engage in all-out war, the conflicts that took place in 1920 were an inevitable escalation of fighting that began in earnest a year earlier. In the end, the events of 1920 were a logical, though unforeseen, consequence of the 1919 prelude. 
Question: How many years did the Russo-Polish War officially last? 
Answer type: Number 
Output (English): 2.