import java.util.Scanner

//
fun _Do() {
    
    val S = scan.next()
    
    // 13
    val set = mutableSetOf<Char>()
    for(c in 'a'..'m') {
        set.add(c)
    }
    
    // 手持ち
    for(c in S) {
        set.remove(c)
    }
    
    //
    when(set.size){
        0 ->
            for(c in 'a'..'m') println(c)
        
        1 ->
            println(set.first())
        
        else ->
            println("Impossible")
    }
}

var scan = Scanner(System.`in`)
fun main( argv : Array<String> ) {
    _Do()
}