fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let s: Vec<char> = s.trim().chars().collect();

    let count: Vec<usize> = ('a'..='z')
        .map(|c| s.iter().filter(|&&d| d == c).count())
        .collect();

    if count.iter().filter(|&&n| n == 2).count() == 6 {
        println!(
            "{}",
            ('a'..='z')
                .nth(count.iter().position(|&n| n == 1).unwrap())
                .unwrap()
        );
    } else {
        println!("Impossible");
    }
}