fn primera_palabra(s: &String) -> usize {
    let b = s.as_bytes();
    for (i, &item) in b.iter().enumerate() {
        if item == b' ' {
            return i;
        }
    }
    s.len()
}

fn main() {
    let s = String::from("Hola Mundo Cruel");
    let pos = primera_palabra(&s);
    let palabra = &s[0..pos];
    println!("{}", palabra);
}