练习：Fibonacci

The first and second Fibonacci numbers are both 1. For n>2, the n'th Fibonacci number is calculated recursively as the sum of the n-1'th and n-2'th Fibonacci numbers.

Write a function fib(n) that calculates the n'th Fibonacci number. When will this function panic?

fn fib(n: u32) -> u32 {
    if n <= 2 {
        // The base case.
        todo!("Implement this")
    } else {
        // The recursive case.
        todo!("Implement this")
    }
}

fn main() {
    let n = 20;
    println!("fib({n}) = {}", fib(n));
}