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();
		int[] cnt=new int[26];
		for(int i=0;i<cs.length;++i) {
			cnt[(int)(cs[i]-'a')]++;
		}
		int c2=0,c1=0,idx=-1;
		for(int i=0;i<cnt.length;++i) {
			if(cnt[i]==2)++c2;
			else if(cnt[i]==1) {
				++c1;
				idx=i;
			}
		}
		if(c2==6&&c1==1) {
			System.out.println((char)('a'+idx));
		}else {
			System.out.println("Impossible");
		}
	}
	
	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
	
}