fn main() {
    let mut n = String::new();
    std::io::stdin().read_line(&mut n).ok();
    let mut n: u64 = n.trim().parse().unwrap();

    for _ in 0..99 {
        let mut next_n = 0;

        while n > 0 {
            next_n += n % 10;
            n /= 10;
        }

        n = next_n;

        if n < 10 {
            break;
        }
    }

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