#include<stdio.h>

int main(void){
	int b[128]={0},i,t=0,x;
	char s[16];
	scanf("%s",s);
	for(i = 0;i < 13;i++){b[s[i]]++;}
	for(i = 97;i <= 122;i++){
		if(b[i] == 2){t++;}
		if(b[i] == 1){x = i;}
	}
	if(t == 6){printf("%c\n",x);}else{printf("Impossible\n");}
	return 0;
}