use std::collections::HashMap;

fn main() {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	let mut h = HashMap::new();
	for c in s.trim().chars() {
		let v = h.entry(c).or_insert(0);
		*v += 1;
		if *v > 2 || h.len() > 7 {
			println!("Impossible");
			return;
		}
	}
	println!("{}", h.iter().find(|&(_, &v)| v == 1).unwrap().0)
}