Let $\operatorname{smpf}(n)$ be the smallest prime factor of $n$.
$\operatorname{smpf}(91)=7$ because $91=7\times 13$ and $\operatorname{smpf}(45)=3$ because $45=3\times 3\times 5$.
Let $S(n)$ be the sum of $\operatorname{smpf}(i)$ for $2 \le i \le n$.
E.g. $S(100)=1257$.
Find $S(2^N)$.