Index of a Fibonacci number

The Fibonacci numbers are $F_1 = 1$, $F_2 = 1$, $F_k = F_{k-1} + F_{k-2}$, that is $1, 1, 2, 3, 5, 8, 13, \ldots$

You are given a natural number $n$ ($n \ge 2$). If $n$ is a Fibonacci number, print its index $k$ ($n = F_k$), otherwise print $0$.

A single integer $n$ ($2 \le n \le 10^{18}$).

A single integer — $k$ or $0$.

Generate Fibonacci numbers one by one while the current one is less than $n$ — a while loop. If it equals $n$ when the loop stops, you found it. In the first example $13 = F_7$; $7$ lies between $5$ and $8$, so the answer is $0$.

Numbers up to $10^{18}$ need long long / int64.