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 mut x = get()
        .as_bytes()
        .iter()
        .cloned()
        .collect::<Vec<_>>();
    x.sort();
    let find = x
        .iter()
        .enumerate()
        .find(|(_,s)| **s == x[0]);
    let ans = match find {
        None => "-1".to_string(),
        Some((i, _)) => {
            x.swap(0, i);
            x.reverse();
            String::from_utf8(x).unwrap()
        }
    };
    println!("{}", ans);
}