#![allow(non_snake_case)]

#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut tokens = $s.split_whitespace();
        input_inner! { tokens, $($r)* }
    };

    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut res = String::new();
            ::std::io::stdin().read_to_string(&mut res).unwrap();
            res
        };
        let mut tokens = s.split_whitespace();
        input_inner! { tokens, $($r)* }
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($tokens:expr)  => {};
    ($tokens:expr,) => {};

    ($tokens:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($tokens, $t);
        input_inner! { $tokens $($r)* }
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($tokens:expr, ( $($t:tt),* )) => {
        ( $(read_value!($tokens, $t)),* )
    };

    ($tokens:expr, [ $t:tt; $len:expr ]) => {
        (0..$len).map(|_| read_value!($tokens, $t)).collect::<Vec<_>>()
    };

    ($tokens:expr, chars) => {
        read_value!($tokens, String).chars().collect::<Vec<_>>()
    };

    ($tokens:expr, usize1) => {
        read_value!($tokens, usize) - 1
    };

    ($tokens:expr, $t:ty) => {
        $tokens.next().unwrap().parse::<$t>().expect("parse error")
    };
}

fn main() {
    input! {
        S: chars,
    }

    let mut cnt = [0; 26];
    for c in S {
        cnt[(c as u8 - b'a') as usize] += 1;
    }

    let n_two = cnt.iter().filter(|&&e| e == 2).count();
    if n_two == 6 {
        let ans = (b'a' + cnt.iter().position(|&e| e == 1).unwrap() as u8) as char;
        println!("{}", ans);
    }
    else {
        println!("Impossible");
    }
}