#include <stdio.h>

int main(void) {
	int al[256] = {0};
	int i;
	int cnt = 0;
	char ac;
	
	for(i = 0; i < 13; i++){
		al[getchar()]++;
	}
	
	for(i = 'a'; i <= 'z'; i++){
		if(al[i] == 0) continue;
		if(al[i] == 1){
			ac = (char)i;
			cnt++;
		}else if(al[i] > 2){
			goto END;
		}
		if(cnt > 1){
END:			printf("Impossible\n");
			return 0;
		}
	}
	
	printf("%c\n", ac);
	
	return 0;
}