import java.util.Scanner

fun duplication(input: List<String>): MutableList<String>{
	var list: MutableList<String> = mutableListOf()
	for(i in 0..input.count() - 2){
		if( !(input[i] in list) ) list.add(input[i])
	}
	return list
}

fun charaCount(input: List<String>, duplicationList: MutableList<String>): MutableList<Int>{
	var countList: MutableList<Int> = mutableListOf()
	for(item in duplicationList){
		var count: Int = 0
		for(i in 0..input.count() - 2){
			if(item == input[i]) count++
		}
		countList.add(count)
	}
	return countList
}

fun run(duplicationList: MutableList<String>, countList: MutableList<Int>){
	var flag: Boolean = false
	var result: MutableList<String> = mutableListOf()
	for(i in 0..countList.count() - 1){
		if(countList[i] > 2){
			flag = false
			break
		} else if(countList[i] == 1){
			result.add(duplicationList[i])
			flag = true
		}
	}
	if(flag == true && result.count() == 1) println(result[0])
	else println("Impossible")
}

fun main(args: Array<String>){
	var input = (Scanner(System. `in`).next()).split(Regex(""))
	var duplicationList = duplication(input)
	var countList = charaCount(input, duplicationList)
	run(duplicationList, countList)
}