// Option, and the type alias expansion that `Result<T>` in base16ct needs.
type MyResult<T> = Result<T, i32>;

fn first_even(a: i32, b: i32) -> Option<i32> {
    if a % 2 == 0 {
        Some(a)
    } else if b % 2 == 0 {
        Some(b)
    } else {
        None
    }
}

fn checked(n: i32) -> MyResult<i32> {
    let v: Option<i32> = first_even(n, n + 1);
    let got: i32 = v.ok_or(-1)?;
    Ok(got * 10)
}

fn main() {
    println!("{:?} {:?}", first_even(2, 3), first_even(1, 3));
    println!("{}", first_even(1, 4).unwrap());
    println!("{}", first_even(1, 3).unwrap_or(-7));
    println!("{:?} {:?}", checked(2), checked(1));
}