fn main() {
    println!("fib(1) = {}", fib(1));
    println!("fib(7) = {}", fib(7));
    println!("fib(19) = {}", fib(19));
}

fn fib(i: u64) -> u64 {
    let (mut a, mut b) = (0, 1);

    for _ in 0..i {
        (a, b) = (b, a + b);
    }

    a
}