import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Scanner;

class Main {
	public static void main(String[] args) throws Exception {
		new Main().run();
	}
	
	void run() {
		Scanner sc=new Scanner(System.in);
		char[] cs=sc.next().toCharArray();
		boolean find=false;
		for(int i=0;i<26;++i) {
			int[] cnt=new int[26];
			for(int j=0;j<cs.length;++j)cnt[(int)(cs[j]-'a')]++;
			cnt[i]++;
			boolean flag=true;
			for(int j=0;j<13;++j) {
				flag&=cnt[j]>=1&&2>=cnt[j];
			}
			int c2=0;
			for(int j=0;j<13;++j)if(cnt[j]==2)++c2;
			flag&=c2==1;
			if(flag) {
				System.out.println((char)('a'+i));
				find=true;
			}
		}
		if(!find)System.out.println("Impossible");
	}
	
	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
	
}