use std::cmp::*;
use std::collections::*;
use std::io::*;
use std::str::FromStr;
fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}
fn main(){
    let s:String = read();
    let t:Vec<char> = s.chars().collect();
    let mut mp:HashMap<char,usize> = HashMap::new();
    for i in &t{
        let cnt = mp.entry(*i).or_insert(0);
        *cnt += 1;
    }
    let mut cnt = 0;
    let mut idx = ' ';
    for (i,j) in &mp{
        if *j == 1usize{
            cnt += 1;
            idx = *i;
        }
    }
    if cnt == 1 && mp.len() == 7{
        println!("{}",idx);
    }else{
        println!("Impossible");
    }
}