use std::collections::HashMap;

fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let s: &str = s.trim();
    let mut c2c: HashMap<char, usize> = HashMap::new();
    s.chars().for_each(|c| {
        if let Some(x) = c2c.get_mut(&c) {
            *x += 1;
        } else {
            c2c.insert(c, 1);
        }
    });

    if c2c.len() != 7 || c2c.iter().filter(|pair| *pair.1 != 2).count() != 1 {
        println!("Impossible");
    } else  {
        println!("{}", c2c.iter().find(|pair| *pair.1 == 1).map(|pair| pair.0).unwrap());
    }
}