fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let s: &str = s.trim();
    const ALPHABET: &str = "abcdefghijklmnopqrstuvwxyz";
    let result = ALPHABET.chars().zip(s.chars())
        .find(|pair| pair.0 != pair.1)
        .map(|pair| format!("{}to{}", pair.0, pair.1))
        .unwrap()
    ;
    println!("{}", result);
}