use std::io::*;

fn main() {
    let mut s: String = String::new();
    std::io::stdin().read_to_string(&mut s).ok();
    let mut itr = s.trim().split_whitespace();
    let mut s: Vec<char> = itr.next().unwrap().chars().collect();
    s.sort();
    s.push('*');
    for i in 0..s.len() {
        print!("a");
        if s[i] != 'a' {
            break;
        }
    }
}