As we see the series 1,4,9,18......
we see that $ 2^{1}$-1 = 1 which is first term
$ 2^{2}$-0 = 4 which is second term
$2^{3}$+1 = 9 third term
$ 2^{4}$+2 = 18 fourth term
$2^{5}$+3 = 35 fifth term
$2^{6}$+4 = 68 sixth term
$2^{7}$+5 = 133 seventh term
$ 2^{8}$+6 = 262 eighth term
so answer is 262