use std::collections::HashMap;

fn main() {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	let l = "abcdefghijklm".chars();
	let mut a = HashMap::new();
	for c in s.trim().chars() {
		*a.entry(c).or_insert(0) += 1;
	}
	match a.len() {
		13 => l.for_each(|c| println!("{}", c)),
		12 => l
			.filter(|&c| !a.contains_key(&c))
			.for_each(|c| println!("{}", c)),
		_ => println!("Impossible"),
	}
}