After a series of Mongol invasions of Korea between 1231 and 1281, Goryeo signed a treaty in favor of the Mongols and became a vassal state. Kublai was declared Khagan of the Mongol Empire in 1260  and established his capital at Khanbaliq  in 1264. Japan at the time was ruled by the Shikken  of the Hōjō clan, who had intermarried with and wrested control from Minamoto no Yoriie, shōgun of the Kamakura shogunate, after his death in 1203. The inner circle of the Hōjō clan had become so preeminent that they no longer consulted the council of the shogunate ), the Imperial Court of Kyoto, or their gokenin vassals, and made their decisions at private meetings in their residences ). The Mongols also made attempts to subjugate the native peoples of Sakhalin—the Ainu, Orok, and Nivkh peoples—from 1260 to 1308.
Question: How many years did the series of Mongol invasions of Korea take place?
Answer: 50