use std::io;
use std::collections::HashMap;

fn process(input: &str) -> Option<char> {
    let char_map = input.chars().fold(HashMap::new(), |mut acc, c| {
        *acc.entry(c).or_insert(0) += 1;
        acc
    });

    if char_map.len() != 7 {
        return None;
    }

    if let Some(x) = char_map.iter().find(|entry| *entry.1 == 1) {
        Some(*x.0)
    } else {
        None
    }
}

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).ok();

    if let Some(x) = process(&input.trim()) {
        println!("{}", x);
    } else {
        println!("Impossible");
    }
}