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從LV. 2升到LV.3要 2^1 本，
從LV. 3升到LV. 4要 2^2 本……
從LV. k升到LV. (k+1)要 2^(k-1) 本……
從LV. 19升到LV. 20要 2^18 本。
所以從LV. 1升到LV. 20要 524287 本。
https://www.wolframalpha.com/input?i=%E2%88%91+2%5Ek+from+k%3D0+to+k%3D18