두 로그 사이 정수 합

$\log _{2} 1 < f ( 1 ) = 1 < \log _{2} 3$ $\log _{2} 2 < f ( 2 ) = 2 < \log _{2} 5$ $\log _{2} 4 < f ( 4 ) = 3 < \log _{2} 9$ $\log _{2} 8 < f ( 8 ) = 4 < \log _{2} 17$ $\log _{2} 16 < f ( 16 ) = 5 < \log _{2} 33$ $\log _{2} 20 < f ( 20 ) = 5 < \log _{2} 41$ $\displaystyle \sum\limits _{n = 1} ^{20} f ( n )$$= 1 + 2 \times 2 + 3 \times 4 + 4 \times 8 + 5 \times 5$$= 74$