use std::io::{Read, stdin};

fn main() {
    let mut buf = String::new();
    stdin().read_to_string(&mut buf).unwrap();
    let mut tok = buf.split_whitespace();
    let mut get = || tok.next().unwrap();
    
    let s = get().as_bytes();
    
    for t in b'A'..b'Z'+1 {
        let c = s
            .iter()
            .filter(|&ch| *ch == t)
            .count();
        if c > 1 {
            println!("NO");
            return;
        }
    }
    println!("YES");
}