Calculating Powers of Two

Taeyoung's hobby is computing powers of two.

Taeyoung knows that $2^{64} = 18\,446\,744\,073\,709\,551\,616$, and he will compute $2^{64}$ by hand, starting from $2^0$ and multiplying by 2 each time.

However, while multiplying by 2, Taeyoung made the mistake of subtracting 1 exactly once. (He makes the mistake only once, and after that he performs the multiplication by 2 correctly.)

For example, he computes $2^1 = 2$ correctly, then makes the absurd mistake of computing $2^2 = 3$.

Then he goes on to compute $2^3 = 6$, $2^4 = 12$, and so on, so the error grows larger and larger.

Given $N$, the value of $2^{64}$ that Taeyoung obtained, find the step where he made his first mistake.

A positive integer $N$ is given.

$N$ is a value that can result when Taeyoung computes $2^{64}$.

Find the step where Taeyoung made his first mistake.

Print $K$ for the case where he computed $2^K = 2^{K-1}$.

• $2 \le N \le 18\,446\,744\,073\,709\,551\,615 = 2^{64} - 1$