Rust CLI app for getting nth fibonacci number.

fibo.rs

use std::io;

fn main() {
    //nth fibonacci number generator
    loop{
        println!("Enter position of fibonacci you want.");

        let mut pos = String::from("");
        io::stdin().read_line(&mut pos)
            .expect("Could not read input.");
        
        let pos_int: isize = match pos.trim().parse() {
            Ok(val) => val,
            Err(_) => continue
        };

        let fibo = get_fibo(pos_int);

        println!("{}", fibo);
    }
}

fn get_fibo(pos: isize) -> isize {
    let mut counter = 0;
    let mut a = 0;
    let mut b = 1;
    let mut fibo = 1;
    while counter < pos - 1 {
        fibo = a + b;
        a = b;
        b = fibo;
        counter = counter + 1;
    }
    fibo
}