#include <stdio.h>

int main(){
	char str[13];
	scanf("%s",str);
	
	int arr[26];
	for(int i = 0;i < 26;i ++){
		arr[i] = i + 97;
	}
	
	int record[26];
	for(int i = 0;i < 26;i ++){
		record[i] = 0;
	}
	for(int i = 0;i < 13;i ++){
		for(int j = 0;j < 26;j ++){
			if((int)str[i] == arr[j]){
				record[j]++;
			}
		}
	}
	
	int count = 0;
	char ans[1];
	for(int i = 0;i < 26;i ++){
		if(record[i] != 0 && record[i] != 2){
			count++;
			ans[0] = arr[i];
		}else if(record[i] > 2){
			printf("Impossible");
			return 0;
		}
	}
	
	if(count == 1){
		printf("%c",ans[0]);
	}else{
		printf("Impossible");
	}
}